XEmacs 21.2.28 "Hermes".

[chise/xemacs-chise.git.1] / man / xemacs / programs.texi

@node Programs, Running, Text, Top
@chapter Editing Programs
@cindex Lisp
@cindex C

  Emacs has many commands designed to understand the syntax of programming
languages such as Lisp and C.  These commands can:

@itemize @bullet
@item
Move over or kill balanced expressions or @dfn{sexps} (@pxref{Lists}).
@item
Move over or mark top-level balanced expressions (@dfn{defuns}, in Lisp;
functions, in C).
@item
Show how parentheses balance (@pxref{Matching}).
@item
Insert, kill, or align comments (@pxref{Comments}).
@item
Follow the usual indentation conventions of the language
(@pxref{Grinding}).
@end itemize

  The commands available for words, sentences, and paragraphs are useful in
editing code even though their canonical application is for editing human
language text.  Most symbols contain words (@pxref{Words}); sentences can
be found in strings and comments (@pxref{Sentences}).  Paragraphs per se
are not present in code, but the paragraph commands are useful anyway,
because Lisp mode and C mode define paragraphs to begin and end at blank
lines (@pxref{Paragraphs}).  Judicious use of blank lines to make the
program clearer also provides interesting chunks of text for the
paragraph commands to work on.

  The selective display feature is useful for looking at the overall
structure of a function (@pxref{Selective Display}).  This feature causes
only the lines that are indented less than a specified amount to appear
on the screen.

@menu
* Program Modes::       Major modes for editing programs.
* Lists::               Expressions with balanced parentheses.
                         There are editing commands to operate on them.
* Defuns::              Each program is made up of separate functions.
                         There are editing commands to operate on them.
* Grinding::            Adjusting indentation to show the nesting.
* Matching::            Insertion of a close-delimiter flashes matching open.
* Comments::            Inserting, illing and aligning comments.
* Balanced Editing::    Inserting two matching parentheses at once, etc.
* Lisp Completion::     Completion on symbol names in Lisp code.
* Documentation::       Getting documentation of functions you plan to call.
* Change Log::          Maintaining a change history for your program.
* Tags::                Go direct to any function in your program in one
                         command.  Tags remembers which file it is in.
* Fortran::             Fortran mode and its special features.
* Asm Mode::            Asm mode and its special features.
@end menu

@node Program Modes, Lists, Programs, Programs
@section Major Modes for Programming Languages

@cindex Lisp mode
@cindex C mode
@cindex Scheme mode
  Emacs has several major modes for the programming languages Lisp, Scheme (a
variant of Lisp), C, Fortran, and Muddle.  Ideally, a major mode should be
implemented for each programming language you might want to edit with
Emacs; but often the mode for one language can serve for other
syntactically similar languages.  The language modes that exist are those
that someone decided to take the trouble to write.

  There are several variants of Lisp mode, which differ in the way they
interface to Lisp execution.  @xref{Lisp Modes}.

  Each of the programming language modes defines the @key{TAB} key to run
an indentation function that knows the indentation conventions of that
language and updates the current line's indentation accordingly.  For
example, in C mode @key{TAB} is bound to @code{c-indent-line}.  @key{LFD}
is normally defined to do @key{RET} followed by @key{TAB}; thus it, too,
indents in a mode-specific fashion.

@kindex DEL
@findex backward-delete-char-untabify
  In most programming languages, indentation is likely to vary from line to
line.  So the major modes for those languages rebind @key{DEL} to treat a
tab as if it were the equivalent number of spaces (using the command
@code{backward-delete-char-untabify}).  This makes it possible to rub out
indentation one column at a time without worrying whether it is made up of
spaces or tabs.  In these modes, use @kbd{C-b C-d} to delete a tab
character before point. 

  Programming language modes define paragraphs to be separated only by
blank lines, so that the paragraph commands remain useful.  Auto Fill mode,
if enabled in a programming language major mode, indents the new lines
which it creates.

@cindex mode hook
@vindex c-mode-hook
@vindex lisp-mode-hook
@vindex emacs-lisp-mode-hook
@vindex lisp-interaction-mode-hook
@vindex scheme-mode-hook
@vindex muddle-mode-hook
  Turning on a major mode calls a user-supplied function called the
@dfn{mode hook}, which is the value of a Lisp variable.  For example,
turning on C mode calls the value of the variable @code{c-mode-hook} if
that value exists and is non-@code{nil}.  Mode hook variables for other
programming language modes include @code{lisp-mode-hook},
@code{emacs-lisp-mode-hook}, @code{lisp-interaction-mode-hook},
@code{scheme-mode-hook}, and @code{muddle-mode-hook}.  The mode hook
function receives no arguments.@refill

@node Lists, Defuns, Program Modes, Programs
@section Lists and Sexps

@cindex Control-Meta
  By convention, Emacs keys for dealing with balanced expressions are
usually @kbd{Control-Meta-} characters.  They tend to be analogous in
function to their @kbd{Control-} and @kbd{Meta-} equivalents.  These commands
are usually thought of as pertaining to expressions in programming
languages, but can be useful with any language in which some sort of
parentheses exist (including English).

@cindex list
@cindex sexp
@cindex expression
  The commands fall into two classes.  Some commands deal only with
@dfn{lists} (parenthetical groupings).  They see nothing except
parentheses, brackets, braces (depending on what must balance in the
language you are working with), and escape characters that might be used
to quote those.

  The other commands deal with expressions or @dfn{sexps}.  The word `sexp'
is derived from @dfn{s-expression}, the term for a symbolic expression in
Lisp.  In Emacs, the notion of `sexp' is not limited to Lisp.  It
refers to an expression in the language  your program is written in.
Each programming language has its own major mode, which customizes the
syntax tables so that expressions in that language count as sexps.

  Sexps typically include symbols, numbers, and string constants, as well
as anything contained in parentheses, brackets, or braces.

  In languages that use prefix and infix operators, such as C, it is not
possible for all expressions to be sexps.  For example, C mode does not
recognize @samp{foo + bar} as an sexp, even though it @i{is} a C expression;
it recognizes @samp{foo} as one sexp and @samp{bar} as another, with the
@samp{+} as punctuation between them.  This is a fundamental ambiguity:
both @samp{foo + bar} and @samp{foo} are legitimate choices for the sexp to
move over if point is at the @samp{f}.  Note that @samp{(foo + bar)} is a
sexp in C mode.

  Some languages have obscure forms of syntax for expressions that nobody
has bothered to make Emacs understand properly.

@c doublewidecommands
@table @kbd
@item C-M-f
Move forward over an sexp (@code{forward-sexp}).
@item C-M-b
Move backward over an sexp (@code{backward-sexp}).
@item C-M-k
Kill sexp forward (@code{kill-sexp}).
@item C-M-u
Move up and backward in list structure (@code{backward-up-list}).
@item C-M-d
Move down and forward in list structure (@code{down-list}).
@item C-M-n
Move forward over a list (@code{forward-list}).
@item C-M-p
Move backward over a list (@code{backward-list}).
@item C-M-t
Transpose expressions (@code{transpose-sexps}).
@item C-M-@@
Put mark after following expression (@code{mark-sexp}).
@end table

@kindex C-M-f
@kindex C-M-b
@findex forward-sexp
@findex backward-sexp
  To move forward over an sexp, use @kbd{C-M-f} (@code{forward-sexp}).  If
the first significant character after point is an opening delimiter
(@samp{(} in Lisp; @samp{(}, @samp{[}, or @samp{@{} in C), @kbd{C-M-f}
moves past the matching closing delimiter.  If the character begins a
symbol, string, or number, @kbd{C-M-f} moves over that.  If the character
after point is a closing delimiter, @kbd{C-M-f} just moves past it.  (This
last is not really moving across an sexp; it is an exception which is
included in the definition of @kbd{C-M-f} because it is as useful a
behavior as anyone can think of for that situation.)@refill

  The command @kbd{C-M-b} (@code{backward-sexp}) moves backward over a
sexp.  The detailed rules are like those above for @kbd{C-M-f}, but with
directions reversed.  If there are any prefix characters (single quote,
back quote, and comma, in Lisp) preceding the sexp, @kbd{C-M-b} moves back
over them as well.

  @kbd{C-M-f} or @kbd{C-M-b} with an argument repeats that operation the
specified number of times; with a negative argument, it moves in the
opposite direction.

In languages such as C where the comment-terminator can be recognized,
the sexp commands move across comments as if they were whitespace.  In
Lisp and other languages where comments run until the end of a line, it
is very difficult to ignore comments when parsing backwards; therefore,
in such languages the sexp commands treat the text of comments as if it
were code.

@kindex C-M-k
@findex kill-sexp
  Killing an sexp at a time can be done with @kbd{C-M-k} (@code{kill-sexp}).
@kbd{C-M-k} kills the characters that @kbd{C-M-f} would move over.

@kindex C-M-n
@kindex C-M-p
@findex forward-list
@findex backward-list
  The @dfn{list commands}, @kbd{C-M-n} (@code{forward-list}) and
@kbd{C-M-p} (@code{backward-list}), move over lists like the sexp
commands but skip over any number of other kinds of sexps (symbols,
strings, etc).  In some situations, these commands are useful because
they usually ignore comments, since the comments usually do not contain
any lists.@refill

@kindex C-M-u
@kindex C-M-d
@findex backward-up-list
@findex down-list
  @kbd{C-M-n} and @kbd{C-M-p} stay at the same level in parentheses, when
that is possible.  To move @i{up} one (or @var{n}) levels, use @kbd{C-M-u}
(@code{backward-up-list}).
@kbd{C-M-u} moves backward up past one unmatched opening delimiter.  A
positive argument serves as a repeat count; a negative argument reverses
direction of motion and also requests repetition, so it moves forward and
up one or more levels.@refill

  To move @i{down} in list structure, use @kbd{C-M-d}
(@code{down-list}).  In Lisp mode, where @samp{(} is the only opening
delimiter, this is nearly the same as searching for a @samp{(}.  An
argument specifies the number of levels of parentheses to go down.

@cindex transposition
@kindex C-M-t
@findex transpose-sexps
@kbd{C-M-t} (@code{transpose-sexps}) drags the previous sexp across
the next one.  An argument serves as a repeat count, and a negative
argument drags backwards (thus canceling out the effect of @kbd{C-M-t} with
a positive argument).  An argument of zero, rather than doing nothing,
transposes the sexps ending after point and the mark.

@kindex C-M-@@
@findex mark-sexp
  To make the region be the next sexp in the buffer, use @kbd{C-M-@@}
(@code{mark-sexp}) which sets the mark at the same place that
@kbd{C-M-f} would move to.  @kbd{C-M-@@} takes arguments like
@kbd{C-M-f}.  In particular, a negative argument is useful for putting
the mark at the beginning of the previous sexp.

  The list and sexp commands' understanding of syntax is completely
controlled by the syntax table.  Any character can, for example, be
declared to be an opening delimiter and act like an open parenthesis.
@xref{Syntax}.

@node Defuns, Grinding, Lists, Programs
@section Defuns
@cindex defuns

  In Emacs, a parenthetical grouping at the top level in the buffer is
called a @dfn{defun}.  The name derives from the fact that most
top-level lists in Lisp are instances of the special form
@code{defun}, but Emacs calls any top-level parenthetical
grouping counts a defun regardless of its contents or
the programming language.  For example, in C, the body of a
function definition is a defun.

@c doublewidecommands
@table @kbd
@item C-M-a
Move to beginning of current or preceding defun
(@code{beginning-of-defun}).
@item C-M-e
Move to end of current or following defun (@code{end-of-defun}).
@item C-M-h
Put region around whole current or following defun (@code{mark-defun}).
@end table

@kindex C-M-a
@kindex C-M-e
@kindex C-M-h
@findex beginning-of-defun
@findex end-of-defun
@findex mark-defun
  The commands to move to the beginning and end of the current defun are
@kbd{C-M-a} (@code{beginning-of-defun}) and @kbd{C-M-e} (@code{end-of-defun}).

   To operate on the current defun, use @kbd{C-M-h} (@code{mark-defun})
which puts point at the beginning and the mark at the end of the current
or next defun.  This is the easiest way to prepare for moving the defun
to a different place.  In C mode, @kbd{C-M-h} runs the function
@code{mark-c-function}, which is almost the same as @code{mark-defun},
but which backs up over the argument declarations, function name, and
returned data type so that the entire C function is inside the region.

@findex compile-defun
To compile and evaluate the current defun, use @kbd{M-x compile-defun}. 
This function prints the results in the minibuffer. If you include an 
argument, it inserts the value in the current buffer after the defun.

  Emacs assumes that any open-parenthesis found in the leftmost column is
the start of a defun.  Therefore, @i{never put an open-parenthesis at the
left margin in a Lisp file unless it is the start of a top level list.
Never put an open-brace or other opening delimiter at the beginning of a
line of C code unless it starts the body of a function.}  The most likely
problem case is when you want an opening delimiter at the start of a line
inside a string.  To avoid trouble, put an escape character (@samp{\} in C
and Emacs Lisp, @samp{/} in some other Lisp dialects) before the opening
delimiter.  It will not affect the contents of the string.

  The original Emacs found defuns by moving upward a
level of parentheses until there were no more levels to go up.  This
required scanning back to the beginning of the buffer for every
function.  To speed this up, Emacs was changed to assume
that any @samp{(} (or other character assigned the syntactic class of
opening-delimiter) at the left margin is the start of a defun.  This
heuristic is nearly always right; however, it mandates the convention 
described above.

@node Grinding, Matching, Defuns, Programs
@section Indentation for Programs
@cindex indentation
@cindex grinding

  The best way to keep a program properly indented (``ground'') is to
use Emacs to re-indent it as you change the program.  Emacs has commands
to indent properly either a single line, a specified number of lines, or
all of the lines inside a single parenthetical grouping.

@menu
* Basic Indent::
* Multi-line Indent::   Commands to reindent many lines at once.
* Lisp Indent::         Specifying how each Lisp function should be indented.
* C Indent::            Choosing an indentation style for C code.
@end menu

@node Basic Indent, Multi-line Indent, Grinding, Grinding
@subsection Basic Program Indentation Commands

@c WideCommands
@table @kbd
@item @key{TAB}
Adjust indentation of current line.
@item @key{LFD}
Equivalent to @key{RET} followed by @key{TAB} (@code{newline-and-indent}).
@end table

@kindex TAB
@findex c-indent-line
@findex lisp-indent-line
  The basic indentation command is @key{TAB}, which gives the current
line the correct indentation as determined from the previous lines.  The
function that @key{TAB} runs depends on the major mode; it is
@code{lisp-indent-line} in Lisp mode, @code{c-indent-line} in C mode,
etc.  These functions understand different syntaxes for different
languages, but they all do about the same thing.  @key{TAB} in any
programming language major mode inserts or deletes whitespace at the
beginning of the current line, independent of where point is in the
line.  If point is inside the whitespace at the beginning of the line,
@key{TAB} leaves it at the end of that whitespace; otherwise, @key{TAB}
leaves point fixed with respect to the characters around it.

  Use @kbd{C-q @key{TAB}} to insert a tab at point.

@kindex LFD
@findex newline-and-indent
  When entering a large amount of new code, use @key{LFD}
(@code{newline-and-indent}), which is equivalent to a @key{RET} followed
by a @key{TAB}.  @key{LFD} creates a blank line, then gives it the
appropriate indentation.

  @key{TAB} indents the second and following lines of the body of a
parenthetical grouping each under the preceding one; therefore, if you
alter one line's indentation to be nonstandard, the lines below tend
to follow it.  This is the right behavior in cases where the standard
result of @key{TAB} does not look good.

  Remember that Emacs assumes that an open-parenthesis, open-brace, or
other opening delimiter at the left margin (including the indentation
routines) is the start of a function.  You should therefore never have
an opening delimiter in column zero that is not the beginning of a
function, not even inside a string.  This restriction is vital for
making the indentation commands fast. @xref{Defuns}, for more
information on this behavior.

@node Multi-line Indent, Lisp Indent, Basic Indent, Grinding
@subsection Indenting Several Lines

  Several commands are available to re-indent several lines of code
which have been altered or moved to a different level in a list
structure.


@table @kbd
@item C-M-q
Re-indent all the lines within one list (@code{indent-sexp}).
@item C-u @key{TAB}
Shift an entire list rigidly sideways so that its first line
is properly indented.
@item C-M-\
Re-indent all lines in the region (@code{indent-region}).
@end table

@kindex C-M-q
@findex indent-sexp
@findex indent-c-exp
 To re-indent the contents of a single list, position point before the
beginning of it and type @kbd{C-M-q}. This key is bound to
@code{indent-sexp} in Lisp mode, @code{indent-c-exp} in C mode, and
bound to other suitable functions in other modes.  The indentation of
the line the sexp starts on is not changed; therefore, only the relative
indentation within the list, and not its position, is changed.  To
correct the position as well, type a @key{TAB} before @kbd{C-M-q}.

@kindex C-u TAB
  If the relative indentation within a list is correct but the
indentation of its beginning is not, go to the line on which the list
begins and type @kbd{C-u @key{TAB}}.  When you give @key{TAB} a numeric
argument, it moves all the lines in the group, starting on the current
line, sideways the same amount that the current line moves.  The command
does not move lines that start inside strings, or C
preprocessor lines when in C mode.

@kindex C-M-\
@findex indent-region
  Another way to specify a range to be re-indented is with point and
mark.  The command @kbd{C-M-\} (@code{indent-region}) applies @key{TAB}
to every line whose first character is between point and mark.

@node Lisp Indent, C Indent, Multi-line Indent, Grinding
@subsection Customizing Lisp Indentation
@cindex customization

  The indentation pattern for a Lisp expression can depend on the function
called by the expression.  For each Lisp function, you can choose among
several predefined patterns of indentation, or define an arbitrary one with
a Lisp program.

  The standard pattern of indentation is as follows: the second line of the
expression is indented under the first argument, if that is on the same
line as the beginning of the expression; otherwise, the second line is
indented underneath the function name.  Each following line is indented
under the previous line whose nesting depth is the same.

@vindex lisp-indent-offset
  If the variable @code{lisp-indent-offset} is non-@code{nil}, it overrides
the usual indentation pattern for the second line of an expression, so that
such lines are always indented @code{lisp-indent-offset} more columns than
the containing list.

@vindex lisp-body-indention
  Certain functions override the standard pattern.  Functions
whose names start with @code{def} always indent the second line by
@code{lisp-body-indention} extra columns beyond the open-parenthesis
starting the expression.

  Individual functions can override the standard pattern in various
ways, according to the @code{lisp-indent-function} property of the
function name.  (Note: @code{lisp-indent-function} was formerly called
@code{lisp-indent-hook}).  There are four possibilities for this
property:

@table @asis
@item @code{nil}
This is the same as no property; the standard indentation pattern is used.
@item @code{defun}
The pattern used for function names that start with @code{def} is used for
this function also.
@item a number, @var{number}
The first @var{number} arguments of the function are
@dfn{distinguished} arguments; the rest are considered the @dfn{body}
of the expression.  A line in the expression is indented according to
whether the first argument on it is distinguished or not.  If the
argument is part of the body, the line is indented @code{lisp-body-indent}
more columns than the open-parenthesis starting the containing
expression.  If the argument is distinguished and is either the first
or second argument, it is indented @i{twice} that many extra columns.
If the argument is distinguished and not the first or second argument,
the standard pattern is followed for that line.
@item a symbol, @var{symbol}
@var{symbol} should be a function name; that function is called to
calculate the indentation of a line within this expression.  The
function receives two arguments:
@table @asis
@item @var{state}
The value returned by @code{parse-partial-sexp} (a Lisp primitive for
indentation and nesting computation) when it parses up to the
beginning of this line.
@item @var{pos}
The position at which the line being indented begins.
@end table
@noindent
It should return either a number, which is the number of columns of
indentation for that line, or a list whose first element is such a
number.  The difference between returning a number and returning a list
is that a number says that all following lines at the same nesting level
should be indented just like this one; a list says that following lines
might call for different indentations.  This makes a difference when the
indentation is computed by @kbd{C-M-q}; if the value is a number,
@kbd{C-M-q} need not recalculate indentation for the following lines
until the end of the list.
@end table

@node C Indent,  , Lisp Indent, Grinding
@subsection Customizing C Indentation

  Two variables control which commands perform C indentation and when.

@vindex c-auto-newline
  If @code{c-auto-newline} is non-@code{nil}, newlines are inserted both
before and after braces that you insert and after colons and semicolons.
Correct C indentation is done on all the lines that are made this way.

@vindex c-tab-always-indent
  If @code{c-tab-always-indent} is non-@code{nil}, the @key{TAB} command
in C mode does indentation only if point is at the left margin or within
the line's indentation.  If there is non-whitespace to the left of point,
@key{TAB} just inserts a tab character in the buffer.  Normally,
this variable is @code{nil}, and @key{TAB} always reindents the current line.

  C does not have anything analogous to particular function names for which
special forms of indentation are desirable.  However, it has a different
need for customization facilities: many different styles of C indentation
are in common use.

  There are six variables you can set to control the style that Emacs C
mode will use.

@table @code
@item c-indent-level
Indentation of C statements within surrounding block.  The surrounding
block's indentation is the indentation of the line on which the
open-brace appears.
@item c-continued-statement-offset
Extra indentation given to a substatement, such as the then-clause of
an @code{if} or body of a @code{while}.
@item c-brace-offset
Extra indentation for lines that start with an open brace.
@item c-brace-imaginary-offset
An open brace following other text is treated as if it were this far
to the right of the start of its line.
@item c-argdecl-indent
Indentation level of declarations of C function arguments.
@item c-label-offset
Extra indentation for a line that is a label, case, or default.
@end table

@vindex c-indent-level
  The variable @code{c-indent-level} controls the indentation for C
statements with respect to the surrounding block.  In the example:

@example
    @{
      foo ();
@end example

@noindent
the difference in indentation between the lines is @code{c-indent-level}.
Its standard value is 2.

If the open-brace beginning the compound statement is not at the beginning
of its line, the @code{c-indent-level} is added to the indentation of the
line, not the column of the open-brace.  For example,

@example
if (losing) @{
  do_this ();
@end example

@noindent
One popular indentation style is that which results from setting
@code{c-indent-level} to 8 and putting open-braces at the end of a line
in this way.  Another popular style prefers to put the open-brace on a
separate line.

@vindex c-brace-imaginary-offset
  In fact, the value of the variable @code{c-brace-imaginary-offset} is
also added to the indentation of such a statement.  Normally this variable
is zero.  Think of this variable as the imaginary position of the open
brace, relative to the first non-blank character on the line.  By setting
the variable to 4 and @code{c-indent-level} to 0, you can get this style:

@example
if (x == y) @{
    do_it ();
    @}
@end example

  When @code{c-indent-level} is zero, the statements inside most braces
line up exactly under the open brace.  An exception are braces in column
zero, like those surrounding a function's body.  The statements inside
those braces are not placed at column zero.  Instead,
@code{c-brace-offset} and @code{c-continued-statement-offset} (see
below) are added to produce a typical offset between brace levels, and
the statements are indented that far.

@vindex c-continued-statement-offset
  @code{c-continued-statement-offset} controls the extra indentation for
a line that starts within a statement (but not within parentheses or
brackets).  These lines are usually statements inside other statements,
like the then-clauses of @code{if} statements and the bodies of
@code{while} statements.  The @code{c-continued-statement-offset}
parameter determines the difference in indentation between the two lines in:

@example
if (x == y)
  do_it ();
@end example

@noindent
The default value for @code{c-continued-statement-offset} is 2.  Some
popular indentation styles correspond to a value of zero for
@code{c-continued-statement-offset}.

@vindex c-brace-offset
  @code{c-brace-offset} is the extra indentation given to a line that
starts with an open-brace.  Its standard value is zero;
compare:

@example
if (x == y)
  @{
@end example

@noindent
with:

@example
if (x == y)
  do_it ();
@end example

@noindent
If you set @code{c-brace-offset} to 4, the first example becomes:

@example
if (x == y)
      @{
@end example

@vindex c-argdecl-indent
  @code{c-argdecl-indent} controls the indentation of declarations of the
arguments of a C function.  It is absolute: argument declarations receive
exactly @code{c-argdecl-indent} spaces.  The standard value is 5 and
results in code like this:

@example
char *
index (string, char)
     char *string;
     int char;
@end example

@vindex c-label-offset
  @code{c-label-offset} is the extra indentation given to a line that
contains a label, a case statement, or a @code{default:} statement.  Its
standard value is @minus{}2 and results in code like this:

@example
switch (c)
  @{
  case 'x':
@end example

@noindent
If @code{c-label-offset} were zero, the same code would be indented as:

@example
switch (c)
  @{
    case 'x':
@end example

@noindent
This example assumes that the other variables above also have their
default values. 

Using the indentation style produced by the default settings of the
variables just discussed and putting open braces on separate lines
produces clear and readable files.  For an example, look at any of the C
source files of XEmacs.

@node Matching, Comments, Grinding, Programs
@section Automatic Display of Matching Parentheses
@cindex matching parentheses
@cindex parentheses

  The Emacs parenthesis-matching feature shows you automatically how
parentheses match in the text.  Whenever a self-inserting character that
is a closing delimiter is typed, the cursor moves momentarily to the
location of the matching opening delimiter, provided that is visible on
the screen.  If it is not on the screen, some text starting with that
opening delimiter is displayed in the echo area.  Either way, you see
the grouping you are closing off. 

  In Lisp, automatic matching applies only to parentheses.  In C, it
also applies to braces and brackets.  Emacs knows which characters to regard
as matching delimiters based on the syntax table set by the major
mode.  @xref{Syntax}.

  If the opening delimiter and closing delimiter are mismatched---as
in @samp{[x)}---the echo area displays a warning message.  The
correct matches are specified in the syntax table.

@vindex blink-matching-paren
@vindex blink-matching-paren-distance
  Two variables control parenthesis matching displays.
@code{blink-matching-paren} turns the feature on or off. The default is 
@code{t} (match display is on); @code{nil} turns it off.
@code{blink-matching-paren-distance} specifies how many characters back
Emacs searches to find a matching opening delimiter.  If the match is
not found in the specified region, scanning stops, and nothing is
displayed.  This prevents wasting lots of time scanning when there is no
match.  The default is 4000.

@node Comments, Balanced Editing, Matching, Programs
@section Manipulating Comments
@cindex comments
@kindex M-;
@cindex indentation
@findex indent-for-comment

  The comment commands insert, kill and align comments.

@c WideCommands
@table @kbd
@item M-;
Insert or align comment (@code{indent-for-comment}).
@item C-x ;
Set comment column (@code{set-comment-column}).
@item C-u - C-x ;
Kill comment on current line (@code{kill-comment}).
@item M-@key{LFD}
Like @key{RET} followed by inserting and aligning a comment
(@code{indent-new-comment-line}).
@end table

  The command that creates a comment is @kbd{Meta-;}
(@code{indent-for-comment}).  If there is no comment already on the
line, a new comment is created and aligned at a specific column called
the @dfn{comment column}.  Emacs creates the comment by inserting the
string at the value of @code{comment-start}; see below.  Point is left
after that string.  If the text of the line extends past the comment
column, indentation is done to a suitable boundary (usually, at least
one space is inserted).  If the major mode has specified a string to
terminate comments, that string is inserted after point, to keep the
syntax valid.

  You can also use @kbd{Meta-;} to align an existing comment.  If a line
already contains the string that starts comments, @kbd{M-;} just moves
point after it and re-indents it to the conventional place.  Exception:
comments starting in column 0 are not moved.

  Some major modes have special rules for indenting certain kinds of
comments in certain contexts.  For example, in Lisp code, comments which
start with two semicolons are indented as if they were lines of code,
instead of at the comment column.  Comments which start with three
semicolons are supposed to start at the left margin.  Emacs understands
these conventions by indenting a double-semicolon comment using @key{TAB}
and by not changing the indentation of a triple-semicolon comment at all.

@example
;; This function is just an example.
;;; Here either two or three semicolons are appropriate.
(defun foo (x)
;;; And now, the first part of the function:
  ;; The following line adds one.
  (1+ x))           ; This line adds one.
@end example

  In C code, a comment preceded on its line by nothing but whitespace
is indented like a line of code.

  Even when an existing comment is properly aligned, @kbd{M-;} is still
useful for moving directly to the start of the comment.

@kindex C-u - C-x ;
@findex kill-comment
  @kbd{C-u - C-x ;} (@code{kill-comment}) kills the comment on the
current line, if there is one.  The indentation before the start of the
comment is killed as well.  If there does not appear to be a comment in
the line, nothing happens.  To reinsert the comment on another line,
move to the end of that line, type first @kbd{C-y}, and then @kbd{M-;}
to realign the comment.  Note that @kbd{C-u - C-x ;} is not a distinct
key; it is @kbd{C-x ;} (@code{set-comment-column}) with a negative
argument.  That command is programmed to call @code{kill-comment} when
called with a negative argument.  However, @code{kill-comment} is a
valid command which you could bind directly to a key if you wanted to.

@subsection Multiple Lines of Comments

@kindex M-LFD
@cindex blank lines
@cindex Auto Fill mode
@findex indent-new-comment-line
  If you are typing a comment and want to continue it on another line,
use the command @kbd{Meta-@key{LFD}} (@code{indent-new-comment-line}),
which terminates the comment you are typing, creates a new blank line
afterward, and begins a new comment indented under the old one.  If
Auto Fill mode is on and you go past the fill column while typing, the 
comment is continued in just this fashion.  If point is
not at the end of the line when you type @kbd{M-@key{LFD}}, the text on
the rest of the line becomes part of the new comment line.

@subsection Options Controlling Comments

@vindex comment-column
@kindex C-x ;
@findex set-comment-column
  The comment column is stored in the variable @code{comment-column}.  You
can explicitly set it to a number.  Alternatively, the command @kbd{C-x ;}
(@code{set-comment-column}) sets the comment column to the column point is
at.  @kbd{C-u C-x ;} sets the comment column to match the last comment
before point in the buffer, and then calls @kbd{Meta-;} to align the
current line's comment under the previous one.  Note that @kbd{C-u - C-x ;}
runs the function @code{kill-comment} as described above.

  @code{comment-column} is a per-buffer variable; altering the variable
affects only the current buffer.  You can also change the default value.
@xref{Locals}.  Many major modes initialize this variable
for the current buffer.

@vindex comment-start-skip
  The comment commands recognize comments based on the regular expression
that is the value of the variable @code{comment-start-skip}.  This regexp
should not match the null string.  It may match more than the comment
starting delimiter in the strictest sense of the word; for example, in C
mode the value of the variable is @code{@t{"/\\*+ *"}}, which matches extra
stars and spaces after the @samp{/*} itself.  (Note that @samp{\\} is
needed in Lisp syntax to include a @samp{\} in the string, which is needed
to deny the first star its special meaning in regexp syntax.  @xref{Regexps}.)

@vindex comment-start
@vindex comment-end
  When a comment command makes a new comment, it inserts the value of
@code{comment-start} to begin it.  The value of @code{comment-end} is
inserted after point and will follow the text you will insert
into the comment.  In C mode, @code{comment-start} has the value
@w{@code{"/* "}} and @code{comment-end} has the value @w{@code{" */"}}.

@vindex comment-multi-line
  @code{comment-multi-line} controls how @kbd{M-@key{LFD}}
(@code{indent-new-comment-line}) behaves when used inside a comment.  If
@code{comment-multi-line} is @code{nil}, as it normally is, then
@kbd{M-@key{LFD}} terminates the comment on the starting line and starts
a new comment on the new following line.  If @code{comment-multi-line}
is not @code{nil}, then @kbd{M-@key{LFD}} sets up the new following line
as part of the same comment that was found on the starting line.  This
is done by not inserting a terminator on the old line and not inserting
a starter on the new line.  In languages where multi-line comments are legal,
the value you choose for this variable is a matter of taste.

@vindex comment-indent-hook
  The variable @code{comment-indent-hook} should contain a function that
is called to compute the indentation for a newly inserted comment or for
aligning an existing comment.  Major modes set this variable differently.
The function is called with no arguments, but with point at the
beginning of the comment, or at the end of a line if a new comment is to
be inserted.  The function should return the column in which the comment
ought to start.  For example, in Lisp mode, the indent hook function
bases its decision on the number of semicolons that begin an existing
comment and on the code in the preceding lines.

@node Balanced Editing, Lisp Completion, Comments, Programs
@section Editing Without Unbalanced Parentheses

@table @kbd
@item M-(
Put parentheses around next sexp(s) (@code{insert-parentheses}).
@item M-)
Move past next close parenthesis and re-indent
(@code{move-over-close-and-reindent}).
@end table

@kindex M-(
@kindex M-)
@findex insert-parentheses
@findex move-over-close-and-reindent
  The commands @kbd{M-(} (@code{insert-parentheses}) and @kbd{M-)}
(@code{move-over-close-and-reindent}) are designed to facilitate a style of
editing which keeps parentheses balanced at all times.  @kbd{M-(} inserts a
pair of parentheses, either together as in @samp{()}, or, if given an
argument, around the next several sexps, and leaves point after the open
parenthesis.  Instead of typing @kbd{( F O O )}, you can type @kbd{M-( F O
O}, which has the same effect except for leaving the cursor before the
close parenthesis.  You can then type @kbd{M-)}, which moves past the
close parenthesis, deletes any indentation preceding it (in this example
there is none), and indents with @key{LFD} after it.

@node Lisp Completion, Documentation, Balanced Editing, Programs
@section Completion for Lisp Symbols
@cindex completion (symbol names)

   Completion usually happens in the minibuffer.  An exception is
completion for Lisp symbol names, which is available in all buffers.

@kindex M-TAB
@findex lisp-complete-symbol
  The command @kbd{M-@key{TAB}} (@code{lisp-complete-symbol}) takes the
partial Lisp symbol before point to be an abbreviation, and compares it
against all non-trivial Lisp symbols currently known to Emacs.  Any
additional characters that they all have in common are inserted at point.
Non-trivial symbols are those that have function definitions, values, or
properties.

  If there is an open-parenthesis immediately before the beginning of
the partial symbol, only symbols with function definitions are considered
as completions.

  If the partial name in the buffer has more than one possible completion
and they have no additional characters in common, a list of all possible
completions is displayed in another window.

@node Documentation, Change Log, Lisp Completion, Programs
@section Documentation Commands

@kindex C-h f
@findex describe-function
@kindex C-h v
@findex describe-variable
  As you edit Lisp code to be run in Emacs, you can use the commands
@kbd{C-h f} (@code{describe-function}) and @kbd{C-h v}
(@code{describe-variable}) to print documentation of functions and
variables you want to call.  These commands use the minibuffer to
read the name of a function or variable to document, and display the
documentation in a window.

  For extra convenience, these commands provide default arguments based on
the code in the neighborhood of point.  @kbd{C-h f} sets the default to the
function called in the innermost list containing point.  @kbd{C-h v} uses
the symbol name around or adjacent to point as its default.

@findex manual-entry
  The @kbd{M-x manual-entry} command gives you access to documentation
on Unix commands, system calls, and libraries.  The command reads a
topic as an argument, and displays the Unix manual page for that topic. 
@code{manual-entry} always searches all 8 sections of the
manual and concatenates all the entries it finds.  For example,
the topic @samp{termcap} finds the description of the termcap library
from section 3, followed by the description of the termcap data base
from section 5.

@node Change Log, Tags, Documentation, Programs
@section Change Logs

@cindex change log
@findex add-change-log-entry
  The Emacs command @kbd{M-x add-change-log-entry} helps you keep a record
of when and why you have changed a program.  It assumes that you have a
file in which you write a chronological sequence of entries describing
individual changes.  The default is to store the change entries in a file
called @file{ChangeLog} in the same directory as the file you are editing.
The same @file{ChangeLog} file therefore records changes for all the files
in a directory.

  A change log entry starts with a header line that contains your name
and the current date.  Except for these header lines, every line in the
change log starts with a tab.  One entry can describe several changes;
each change starts with a line starting with a tab and a star.  @kbd{M-x
add-change-log-entry} visits the change log file and creates a new entry
unless the most recent entry is for today's date and your name.  In
either case, it adds a new line to start the description of another
change just after the header line of the entry.  When @kbd{M-x
add-change-log-entry} is finished, all is prepared for you to edit in
the description of what you changed and how.  You must then save the
change log file yourself.

  The change log file is always visited in Indented Text mode, which means
that @key{LFD} and auto-filling indent each new line like the previous
line.  This is convenient for entering the contents of an entry, which must
be indented.  @xref{Text Mode}.

  Here is an example of the formatting conventions used in the change log
for Emacs:

@smallexample
Wed Jun 26 19:29:32 1985  Richard M. Stallman  (rms at mit-prep)

        * xdisp.c (try_window_id):
        If C-k is done at end of next-to-last line,
        this fn updates window_end_vpos and cannot leave
        window_end_pos nonnegative (it is zero, in fact).
        If display is preempted before lines are output,
        this is inconsistent.  Fix by setting
        blank_end_of_window to nonzero.

Tue Jun 25 05:25:33 1985  Richard M. Stallman  (rms at mit-prep)

        * cmds.c (Fnewline):
        Call the auto fill hook if appropriate.

        * xdisp.c (try_window_id):
        If point is found by compute_motion after xp, record that
        permanently.  If display_text_line sets point position wrong
        (case where line is killed, point is at eob and that line is
        not displayed), set it again in final compute_motion.
@end smallexample

@node Tags, Fortran, Change Log, Programs
@section Tags Tables
@cindex tags table

  A @dfn{tags table} is a description of how a multi-file program is
broken up into files.  It lists the names of the component files and the
names and positions of the functions (or other named subunits) in each
file.  Grouping the related files makes it possible to search or replace
through all the files with one command.  Recording the function names
and positions makes possible the @kbd{M-.} command which finds the
definition of a function by looking up which of the files it is in.

  Tags tables are stored in files called @dfn{tags table files}.  The
conventional name for a tags table file is @file{TAGS}.

  Each entry in the tags table records the name of one tag, the name of the
file that the tag is defined in (implicitly), and the position in that file
of the tag's definition.

  Just what names from the described files are recorded in the tags table
depends on the programming language of the described file.  They
normally include all functions and subroutines, and may also include
global variables, data types, and anything else convenient.  Each name
recorded is called a @dfn{tag}.

@menu
* Tag Syntax::          Tag syntax for various types of code and text files.
* Create Tags Table::   Creating a tags table with @code{etags}.
* Etags Regexps::       Create arbitrary tags using regular expressions.
* Select Tags Table::   How to visit a tags table.
* Find Tag::            Commands to find the definition of a specific tag.
* Tags Search::         Using a tags table for searching and replacing.
* List Tags::           Listing and finding tags defined in a file.
@end menu

@node Tag Syntax, Create Tags Table, Tags, Tags
@subsection Source File Tag Syntax

  Here is how tag syntax is defined for the most popular languages:

@itemize @bullet
@item
In C code, any C function or typedef is a tag, and so are definitions of
@code{struct}, @code{union} and @code{enum}.  You can tag function
declarations and external variables in addition to function definitions
by giving the @samp{--declarations} option to @code{etags}.
@code{#define} macro definitions and @code{enum} constants are also
tags, unless you specify @samp{--no-defines} when making the tags table.
Similarly, global variables are tags, unless you specify
@samp{--no-globals}.  Use of @samp{--no-globals} and @samp{--no-defines}
can make the tags table file much smaller.

@item
In C++ code, in addition to all the tag constructs of C code, member
functions are also recognized, and optionally member variables if you
use the @samp{--members} option.  Tags for variables and functions in
classes are named @samp{@var{class}::@var{variable}} and
@samp{@var{class}::@var{function}}.  @code{operator} functions tags are
named, for example @samp{operator+}.

@item
In Java code, tags include all the constructs recognized in C++, plus
the @code{interface}, @code{extends} and @code{implements} constructs.
Tags for variables and functions in classes are named
@samp{@var{class}.@var{variable}} and @samp{@var{class}.@var{function}}.

@item
In La@TeX{} text, the argument of any of the commands @code{\chapter},
@code{\section}, @code{\subsection}, @code{\subsubsection},
@code{\eqno}, @code{\label}, @code{\ref}, @code{\cite}, @code{\bibitem},
@code{\part}, @code{\appendix}, @code{\entry}, or @code{\index}, is a
tag.@refill

Other commands can make tags as well, if you specify them in the
environment variable @code{TEXTAGS} before invoking @code{etags}.  The
value of this environment variable should be a colon-separated list of
command names.  For example,

@example
TEXTAGS="def:newcommand:newenvironment"
export TEXTAGS
@end example

@noindent
specifies (using Bourne shell syntax) that the commands @samp{\def},
@samp{\newcommand} and @samp{\newenvironment} also define tags.

@item
In Lisp code, any function defined with @code{defun}, any variable
defined with @code{defvar} or @code{defconst}, and in general the first
argument of any expression that starts with @samp{(def} in column zero, is
a tag.

@item
In Scheme code, tags include anything defined with @code{def} or with a
construct whose name starts with @samp{def}.  They also include variables
set with @code{set!} at top level in the file.
@end itemize

  Several other languages are also supported:

@itemize @bullet

@item
In Ada code, functions, procedures, packages, tasks, and types are
tags.  Use the @samp{--packages-only} option to create tags for packages
only.

@item
In assembler code, labels appearing at the beginning of a line,
followed by a colon, are tags.

@item
In Bison or Yacc input files, each rule defines as a tag the nonterminal
it constructs.  The portions of the file that contain C code are parsed
as C code.

@item
In Cobol code, tags are paragraph names; that is, any word starting in
column 8 and followed by a period.

@item
In Erlang code, the tags are the functions, records, and macros defined
in the file.

@item
In Fortran code, functions, subroutines and blockdata are tags.

@item
In Objective C code, tags include Objective C definitions for classes,
class categories, methods, and protocols.

@item
In Pascal code, the tags are the functions and procedures defined in
the file.

@item
In Perl code, the tags are the procedures defined by the @code{sub},
@code{my} and @code{local} keywords.  Use @samp{--globals} if you want
to tag global variables.

@item
In Postscript code, the tags are the functions.

@item
In Prolog code, a tag name appears at the left margin.

@item
In Python code, @code{def} or @code{class} at the beginning of a line
generate a tag.
@end itemize

  You can also generate tags based on regexp matching 
(@pxref{Etags Regexps}) to handle other formats and languages.

@node Create Tags Table, Etags Regexps, Tag Syntax, Tags
@subsection Creating Tags Tables
@cindex @code{etags} program

  The @code{etags} program is used to create a tags table file.  It knows
the syntax of several languages, as described in
@iftex
the previous section.
@end iftex
@ifinfo
@ref{Tag Syntax}.
@end ifinfo
Here is how to run @code{etags}:

@example
etags @var{inputfiles}@dots{}
@end example

@noindent
The @code{etags} program reads the specified files, and writes a tags
table named @file{TAGS} in the current working directory.  You can
intermix compressed and plain text source file names.  @code{etags}
knows about the most common compression formats, and does the right
thing.  So you can compress all your source files and have @code{etags}
look for compressed versions of its file name arguments, if it does not
find uncompressed versions.  Under MS-DOS, @code{etags} also looks for
file names like @samp{mycode.cgz} if it is given @samp{mycode.c} on the
command line and @samp{mycode.c} does not exist.

  @code{etags} recognizes the language used in an input file based on
its file name and contents.  You can specify the language with the
@samp{--language=@var{name}} option, described below.

  If the tags table data become outdated due to changes in the files
described in the table, the way to update the tags table is the same way it
was made in the first place.  It is not necessary to do this often.

  If the tags table fails to record a tag, or records it for the wrong
file, then Emacs cannot possibly find its definition.  However, if the
position recorded in the tags table becomes a little bit wrong (due to
some editing in the file that the tag definition is in), the only
consequence is a slight delay in finding the tag.  Even if the stored
position is very wrong, Emacs will still find the tag, but it must
search the entire file for it.

  So you should update a tags table when you define new tags that you want
to have listed, or when you move tag definitions from one file to another,
or when changes become substantial.  Normally there is no need to update
the tags table after each edit, or even every day.

  One tags table can effectively include another.  Specify the included
tags file name with the @samp{--include=@var{file}} option when creating
the file that is to include it.  The latter file then acts as if it
contained all the files specified in the included file, as well as the
files it directly contains.

  If you specify the source files with relative file names when you run
@code{etags}, the tags file will contain file names relative to the
directory where the tags file was initially written.  This way, you can
move an entire directory tree containing both the tags file and the
source files, and the tags file will still refer correctly to the source
files.

  If you specify absolute file names as arguments to @code{etags}, then
the tags file will contain absolute file names.  This way, the tags file
will still refer to the same files even if you move it, as long as the
source files remain in the same place.  Absolute file names start with
@samp{/}, or with @samp{@var{device}:/} on MS-DOS and MS-Windows.

  When you want to make a tags table from a great number of files, you
may have problems listing them on the command line, because some systems
have a limit on its length.  The simplest way to circumvent this limit
is to tell @code{etags} to read the file names from its standard input,
by typing a dash in place of the file names, like this:

@smallexample
find . -name "*.[chCH]" -print | etags -
@end smallexample

  Use the option @samp{--language=@var{name}} to specify the language
explicitly.  You can intermix these options with file names; each one
applies to the file names that follow it.  Specify
@samp{--language=auto} to tell @code{etags} to resume guessing the
language from the file names and file contents.  Specify
@samp{--language=none} to turn off language-specific processing
entirely; then @code{etags} recognizes tags by regexp matching alone
(@pxref{Etags Regexps}).

  @samp{etags --help} prints the list of the languages @code{etags}
knows, and the file name rules for guessing the language. It also prints
a list of all the available @code{etags} options, together with a short
explanation.

@node Etags Regexps, Select Tags Table, Create Tags Table, Tags
@subsection Etags Regexps

  The @samp{--regex} option provides a general way of recognizing tags
based on regexp matching.  You can freely intermix it with file names.
Each @samp{--regex} option adds to the preceding ones, and applies only
to the following files.  The syntax is:

@smallexample
--regex=/@var{tagregexp}[/@var{nameregexp}]/
@end smallexample

@noindent
where @var{tagregexp} is used to match the lines to tag.  It is always
anchored, that is, it behaves as if preceded by @samp{^}.  If you want
to account for indentation, just match any initial number of blanks by
beginning your regular expression with @samp{[ \t]*}.  In the regular
expressions, @samp{\} quotes the next character, and @samp{\t} stands
for the tab character.  Note that @code{etags} does not handle the other
C escape sequences for special characters.

@cindex interval operator (in regexps)
  The syntax of regular expressions in @code{etags} is the same as in
Emacs, augmented with the @dfn{interval operator}, which works as in
@code{grep} and @code{ed}.  The syntax of an interval operator is
@samp{\@{@var{m},@var{n}\@}}, and its meaning is to match the preceding
expression at least @var{m} times and up to @var{n} times.

  You should not match more characters with @var{tagregexp} than that
needed to recognize what you want to tag.  If the match is such that
more characters than needed are unavoidably matched by @var{tagregexp}
(as will usually be the case), you should add a @var{nameregexp}, to
pick out just the tag.  This will enable Emacs to find tags more
accurately and to do completion on tag names more reliably.  You can
find some examples below.

  The option @samp{--ignore-case-regex} (or @samp{-c}) is like
@samp{--regex}, except that the regular expression provided will be
matched without regard to case, which is appropriate for various
programming languages.

  The @samp{-R} option deletes all the regexps defined with
@samp{--regex} options.  It applies to the file names following it, as
you can see from the following example:

@smallexample
etags --regex=/@var{reg1}/ voo.doo --regex=/@var{reg2}/ \
    bar.ber -R --lang=lisp los.er
@end smallexample

@noindent
Here @code{etags} chooses the parsing language for @file{voo.doo} and
@file{bar.ber} according to their contents.  @code{etags} also uses
@var{reg1} to recognize additional tags in @file{voo.doo}, and both
@var{reg1} and @var{reg2} to recognize additional tags in
@file{bar.ber}.  @code{etags} uses the Lisp tags rules, and no regexp
matching, to recognize tags in @file{los.er}.

  A regular expression can be bound to a given language, by prepending
it with @samp{@{lang@}}.  When you do this, @code{etags} will use the
regular expression only for files of that language.  @samp{etags --help}
prints the list of languages recognised by @code{etags}.  The following
example tags the @code{DEFVAR} macros in the Emacs source files.
@code{etags} applies this regular expression to C files only:

@smallexample
--regex='@{c@}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/'
@end smallexample

@noindent
This feature is particularly useful when storing a list of regular
expressions in a file.  The following option syntax instructs
@code{etags} to read two files of regular expressions.  The regular
expressions contained in the second file are matched without regard to
case.

@smallexample
--regex=@@first-file --ignore-case-regex=@@second-file
@end smallexample

@noindent
A regex file contains one regular expressions per line.  Empty lines,
and lines beginning with space or tab are ignored.  When the first
character in a line is @samp{@@}, @code{etags} assumes that the rest of
the line is the name of a file of regular expressions.  This means that
such files can be nested.  All the other lines are taken to be regular
expressions.  For example, one can create a file called
@samp{emacs.tags} with the following contents (the first line in the
file is a comment):

@smallexample
        -- This is for GNU Emacs source files
@{c@}/[ \t]*DEFVAR_[A-Z_ \t(]+"\([^"]+\)"/\1/
@end smallexample

@noindent
and then use it like this:

@smallexample
etags --regex=@@emacs.tags *.[ch] */*.[ch]
@end smallexample

  Here are some more examples.  The regexps are quoted to protect them
from shell interpretation.

@itemize @bullet

@item
Tag Octave files:

@smallexample
etags --language=none \
      --regex='/[ \t]*function.*=[ \t]*\([^ \t]*\)[ \t]*(/\1/' \
      --regex='/###key \(.*\)/\1/' \
      --regex='/[ \t]*global[ \t].*/' \
      *.m
@end smallexample

@noindent
Note that tags are not generated for scripts so that you have to add a
line by yourself of the form `###key <script-name>' if you want to jump
to it.

@item
Tag Tcl files:

@smallexample
etags --language=none --regex='/proc[ \t]+\([^ \t]+\)/\1/' *.tcl
@end smallexample

@item
Tag VHDL files:

@smallexample
--language=none \
--regex='/[ \t]*\(ARCHITECTURE\|CONFIGURATION\) +[^ ]* +OF/' \
--regex='/[ \t]*\(ATTRIBUTE\|ENTITY\|FUNCTION\|PACKAGE\
\( BODY\)?\|PROCEDURE\|PROCESS\|TYPE\)[ \t]+\([^ \t(]+\)/\3/'
@end smallexample
@end itemize

@node Select Tags Table, Find Tag, Etags Regexps, Tags
@subsection Selecting a Tags Table

@vindex tag-table-alist
   At any time Emacs has one @dfn{selected} tags table, and all the commands
for working with tags tables use the selected one.  To select a tags table,
use the variable @code{tag-table-alist}.

The value of @code{tag-table-alist} is a list that determines which
@code{TAGS} files should be active for a given buffer.  This is not
really an association list, in that all elements are checked.  The car
of each element of this list is a pattern against which the buffers file
name is compared; if it matches, then the cdr of the list should be the
name of the tags table to use.  If more than one element of this list
matches the buffers file name, all of the associated tags tables are
used.  Earlier ones are searched first.

If the car of elements of this list are strings, they are treated
as regular-expressions against which the file is compared (like the
@code{auto-mode-alist}).  If they are not strings, they are evaluated.
If they evaluate to non-@code{nil}, the current buffer is considered to
match.

If the cdr of the elements of this list are strings, they are
assumed to name a tags file.  If they name a directory, the string
@file{tags} is appended to them to get the file name.  If they are not 
strings, they are evaluated and must return an appropriate string.

For example:

@example
  (setq tag-table-alist
        '(("/usr/src/public/perl/" . "/usr/src/public/perl/perl-3.0/")
          ("\\.el$" . "/usr/local/emacs/src/")
          ("/jbw/gnu/" . "/usr15/degree/stud/jbw/gnu/")
          ("" . "/usr/local/emacs/src/")
          ))
@end example

The example defines the tags table alist in the following way:
 
@itemize @bullet
@item
Anything in the directory @file{/usr/src/public/perl/} 
should use the @file{TAGS} file @file{/usr/src/public/perl/perl-3.0/TAGS}. 
@item
Files ending in @file{.el} should use the @file{TAGS} file
@file{/usr/local/emacs/src/TAGS}. 
@item
Anything in or below the directory @file{/jbw/gnu/} should use the 
@file{TAGS} file @file{/usr15/degree/stud/jbw/gnu/TAGS}.  
@end itemize

If you had a file called @file{/usr/jbw/foo.el}, it would use both
@file{TAGS} files, @* @file{/usr/local/emacs/src/TAGS} and
@file{/usr15/degree/stud/jbw/gnu/TAGS} (in that order), because it
matches both patterns.

If the buffer-local variable @code{buffer-tag-table} is set, it names a
tags table that is searched before all others when @code{find-tag} is
executed from this buffer.

If there is a file called @file{TAGS} in the same directory as the file
in question, then that tags file will always be used as well (after the
@code{buffer-tag-table} but before the tables specified by this list).

If the variable @code{tags-file-name} is set, the @file{TAGS} file it names
will apply to all buffers (for backwards compatibility.)  It is searched
first.

@vindex tags-always-build-completion-table
If the value of the variable @code{tags-always-build-completion-table}
is @code{t}, the tags file will always be added to the completion table
without asking first, regardless of the size of the tags file.

@vindex tags-file-name
@findex visit-tags-table
The function @kbd{M-x visit-tags-table}, is largely made obsolete by
the variable @code{tag-table-alist}, tells tags commands to use the tags
table file @var{file} first.  The @var{file} should be the name of a
file created with the @code{etags} program.  A directory name is also
acceptable; it means the file @file{TAGS} in that directory.  The
function only stores the file name you provide in the variable
@code{tags-file-name}.  Emacs does not actually read in the tags table
contents until you try to use them.  You can set the variable explicitly
instead of using @code{visit-tags-table}.  The value of the variable
@code{tags-file-name} is the name of the tags table used by all buffers.
This is for backward compatibility, and is largely supplanted by the
variable @code{tag-table-alist}.
 
@node Find Tag, Tags Search, Select Tags Table, Tags
@subsection Finding a Tag

  The most important thing that a tags table enables you to do is to find
the definition of a specific tag.

@table @kbd
@item M-.@: @var{tag &optional other-window}
Find first definition of @var{tag} (@code{find-tag}).
@item C-u M-.
Find next alternate definition of last tag specified.
@item C-x 4 . @var{tag}
Find first definition of @var{tag}, but display it in another window
(@code{find-tag-other-window}).
@end table

@kindex M-.
@findex find-tag
  @kbd{M-.}@: (@code{find-tag}) is the command to find the definition of
a specified tag.  It searches through the tags table for that tag, as a
string, then uses the tags table information to determine the file in
which the definition is used and the approximate character position of
the definition in the file.  Then @code{find-tag} visits the file,
moves point to the approximate character position, and starts searching
ever-increasing distances away for the text that should appear at
the beginning of the definition.

  If an empty argument is given (by typing @key{RET}), the sexp in the
buffer before or around point is used as the name of the tag to find.
@xref{Lists}, for information on sexps.

  The argument to @code{find-tag} need not be the whole tag name; it can
be a substring of a tag name.  However, there can be many tag names
containing the substring you specify.  Since @code{find-tag} works by
searching the text of the tags table, it finds the first tag in the table
that the specified substring appears in.  To find other tags that match
the substring, give @code{find-tag} a numeric argument, as in @kbd{C-u
M-.}.  This does not read a tag name, but continues searching the tag
table's text for another tag containing the same substring last used.
If your keyboard has a real @key{META} key, @kbd{M-0 M-.}@: is an easier
alternative to @kbd{C-u M-.}.

If the optional second argument @var{other-window} is non-@code{nil}, it uses
another window to display the tag.
Multiple active tags tables and completion are supported.

Variables of note include the following:

@vindex tag-table-alist
@vindex tags-file-name
@vindex tags-build-completion-table
@vindex buffer-tag-table
@vindex make-tags-files-invisible
@vindex tag-mark-stack-max

@table @kbd
@item tag-table-alist
Controls which tables apply to which buffers.
@item tags-file-name
Stores a default tags table.
@item tags-build-completion-table
Controls completion behavior.
@item buffer-tag-table
Specifies a buffer-local table.
@item make-tags-files-invisible
Sets whether tags tables should be very hidden.
@item tag-mark-stack-max
Specifies how many tags-based hops to remember.
@end table

@kindex C-x 4 .
@findex find-tag-other-window
  Like most commands that can switch buffers, @code{find-tag} has another
similar command that displays the new buffer in another window.  @kbd{C-x 4
.}@: invokes the function @code{find-tag-other-window}.  (This key sequence
ends with a period.)

  Emacs comes with a tags table file @file{TAGS} (in the directory
containing Lisp libraries) that includes all the Lisp libraries and all
the C sources of Emacs.  By specifying this file with @code{visit-tags-table}
and then using @kbd{M-.}@: you can quickly look at the source of any Emacs
function.

@node Tags Search, List Tags, Find Tag, Tags
@subsection Searching and Replacing with Tags Tables

  The commands in this section visit and search all the files listed in the
selected tags table, one by one.  For these commands, the tags table serves
only to specify a sequence of files to search.  A related command is
@kbd{M-x grep} (@pxref{Compilation}).

@table @kbd
@item M-x tags-search @key{RET} @var{regexp} @key{RET}
Search for @var{regexp} through the files in the selected tags
table.
@item M-x tags-query-replace @key{RET} @var{regexp} @key{RET} @var{replacement} @key{RET}
Perform a @code{query-replace-regexp} on each file in the selected tags table.
@item M-,
Restart one of the commands above, from the current location of point
(@code{tags-loop-continue}).
@end table

@findex tags-search
  @kbd{M-x tags-search} reads a regexp using the minibuffer, then
searches for matches in all the files in the selected tags table, one
file at a time.  It displays the name of the file being searched so you
can follow its progress.  As soon as it finds an occurrence,
@code{tags-search} returns.

@kindex M-,
@findex tags-loop-continue
  Having found one match, you probably want to find all the rest.  To find
one more match, type @kbd{M-,} (@code{tags-loop-continue}) to resume the
@code{tags-search}.  This searches the rest of the current buffer, followed
by the remaining files of the tags table.@refill

@findex tags-query-replace
  @kbd{M-x tags-query-replace} performs a single
@code{query-replace-regexp} through all the files in the tags table.  It
reads a regexp to search for and a string to replace with, just like
ordinary @kbd{M-x query-replace-regexp}.  It searches much like @kbd{M-x
tags-search}, but repeatedly, processing matches according to your
input.  @xref{Replace}, for more information on query replace.

  It is possible to get through all the files in the tags table with a
single invocation of @kbd{M-x tags-query-replace}.  But often it is
useful to exit temporarily, which you can do with any input event that
has no special query replace meaning.  You can resume the query replace
subsequently by typing @kbd{M-,}; this command resumes the last tags
search or replace command that you did.

  The commands in this section carry out much broader searches than the
@code{find-tag} family.  The @code{find-tag} commands search only for
definitions of tags that match your substring or regexp.  The commands
@code{tags-search} and @code{tags-query-replace} find every occurrence
of the regexp, as ordinary search commands and replace commands do in
the current buffer.

  These commands create buffers only temporarily for the files that they
have to search (those which are not already visited in Emacs buffers).
Buffers in which no match is found are quickly killed; the others
continue to exist.

  It may have struck you that @code{tags-search} is a lot like
@code{grep}.  You can also run @code{grep} itself as an inferior of
Emacs and have Emacs show you the matching lines one by one.  This works
much like running a compilation; finding the source locations of the
@code{grep} matches works like finding the compilation errors.
@xref{Compilation}.

  If you wish to process all the files in a selected tags table, but
@kbd{M-x tags-search} and @kbd{M-x tags-query-replace} are not giving
you the desired result, you can use @kbd{M-x next-file}.

@table @kbd
@item C-u M-x next-file
With a numeric argument, regardless of its value, visit the first
file in the tags table and prepare to advance sequentially by files.
@item M-x next-file
Visit the next file in the selected tags table.
@end table

@node List Tags,  , Tags Search, Tags
@subsection Tags Table Inquiries

@table @kbd
@item M-x list-tags
Display a list of the tags defined in a specific program file.
@item M-x tags-apropos
Display a list of all tags matching a specified regexp.
@end table

@findex list-tags
  @kbd{M-x list-tags} reads the name of one of the files described by the
selected tags table, and displays a list of all the tags defined in that
file.  The ``file name'' argument is really just a string to compare
against the names recorded in the tags table; it is read as a string rather
than a file name.  Therefore, completion and defaulting are not
available, and you must enter the string the same way it appears in the tag
table.  Do not include a directory as part of the file name unless the file
name recorded in the tags table contains that directory.

@findex tags-apropos
  @kbd{M-x tags-apropos} is like @code{apropos} for tags.  It reads a regexp,
then finds all the tags in the selected tags table whose entries match that
regexp, and displays the tag names found.

@node Fortran, Asm Mode, Tags, Programs
@section Fortran Mode
@cindex Fortran mode

  Fortran mode provides special motion commands for Fortran statements and
subprograms, and indentation commands that understand Fortran conventions
of nesting, line numbers, and continuation statements.

  Special commands for comments are provided because Fortran comments are
unlike those of other languages.

  Built-in abbrevs optionally save typing when you insert Fortran keywords.

@findex fortran-mode
  Use @kbd{M-x fortran-mode} to switch to this major mode.  Doing so calls
the value of @code{fortran-mode-hook} as a function of no arguments if
that variable has a non-@code{nil} value.

@menu
* Motion: Fortran Motion.     Moving point by statements or subprograms.
* Indent: Fortran Indent.     Indentation commands for Fortran.
* Comments: Fortran Comments. Inserting and aligning comments.
* Columns: Fortran Columns.   Measuring columns for valid Fortran.
* Abbrev: Fortran Abbrev.     Built-in abbrevs for Fortran keywords.
@end menu

  Fortran mode was contributed by Michael Prange.

@node Fortran Motion, Fortran Indent, Fortran, Fortran
@subsection Motion Commands

  Fortran mode provides special commands to move by subprograms (functions
and subroutines) and by statements.  There is also a command to put the
region around one subprogram, which is convenient for killing it or moving it.

@kindex C-M-a (Fortran mode)
@kindex C-M-e (Fortran mode)
@kindex C-M-h (Fortran mode)
@kindex C-c C-p (Fortran mode)
@kindex C-c C-n (Fortran mode)
@findex beginning-of-fortran-subprogram
@findex end-of-fortran-subprogram
@findex mark-fortran-subprogram
@findex fortran-previous-statement
@findex fortran-next-statement

@table @kbd
@item C-M-a
Move to beginning of subprogram@*
(@code{beginning-of-fortran-subprogram}).
@item C-M-e
Move to end of subprogram (@code{end-of-fortran-subprogram}).
@item C-M-h
Put point at beginning of subprogram and mark at end
(@code{mark-fortran-subprogram}).
@item C-c C-n
Move to beginning of current or next statement
(@code{fortran-next-@*statement}).
@item C-c C-p
Move to beginning of current or previous statement
(@code{fortran-@*previous-statement}).
@end table

@node Fortran Indent, Fortran Comments, Fortran Motion, Fortran
@subsection Fortran Indentation

  Special commands and features are available for indenting Fortran
code.  They make sure various syntactic entities (line numbers, comment line
indicators, and continuation line flags) appear in the columns that are
required for standard Fortran.

@menu
* Commands: ForIndent Commands. Commands for indenting Fortran.
* Numbers:  ForIndent Num.      How line numbers auto-indent.
* Conv:     ForIndent Conv.     Conventions you must obey to avoid trouble.
* Vars:     ForIndent Vars.     Variables controlling Fortran indent style.
@end menu

@node ForIndent Commands, ForIndent Num, Fortran Indent, Fortran Indent
@subsubsection Fortran Indentation Commands

@table @kbd
@item @key{TAB}
Indent the current line (@code{fortran-indent-line}).
@item M-@key{LFD}
Break the current line and set up a continuation line.
@item C-M-q
Indent all the lines of the subprogram point is in
(@code{fortran-indent-subprogram}).
@end table

@findex fortran-indent-line
  @key{TAB} is redefined by Fortran mode to reindent the current line for
Fortran (@code{fortran-indent-line}).  Line numbers and continuation
markers are indented to their required columns, and the body of the
statement is independently indented, based on its nesting in the program.

@kindex C-M-q (Fortran mode)
@findex fortran-indent-subprogram
  The key @kbd{C-M-q} is redefined as @code{fortran-indent-subprogram}, a
command that reindents all the lines of the Fortran subprogram (function or
subroutine) containing point.

@kindex M-LFD (Fortran mode)
@findex fortran-split-line
  The key @kbd{M-@key{LFD}} is redefined as @code{fortran-split-line}, a
command to split a line in the appropriate fashion for Fortran.  In a
non-comment line, the second half becomes a continuation line and is
indented accordingly.  In a comment line, both halves become separate
comment lines.

@node ForIndent Num, ForIndent Conv, ForIndent Commands, Fortran Indent
@subsubsection Line Numbers and Continuation

  If a number is the first non-whitespace in the line, it is assumed to be
a line number and is moved to columns 0 through 4.  (Columns are always
counted from 0 in XEmacs.)  If the text on the line starts with the
conventional Fortran continuation marker @samp{$}, it is moved to column 5.
If the text begins with any non whitespace character in column 5, it is
assumed to be an unconventional continuation marker and remains in column
5.

@vindex fortran-line-number-indent
  Line numbers of four digits or less are normally indented one space.
This amount is controlled by the variable @code{fortran-line-number-indent},
which is the maximum indentation a line number can have.  Line numbers
are indented to right-justify them to end in column 4 unless that would
require more than the maximum indentation.  The default value of the
variable is 1.

@vindex fortran-electric-line-number
  Simply inserting a line number is enough to indent it according to these
rules.  As each digit is inserted, the indentation is recomputed.  To turn
off this feature, set the variable @code{fortran-electric-line-number} to
@code{nil}.  Then inserting line numbers is like inserting anything else.

@node ForIndent Conv, ForIndent Vars, ForIndent Num, Fortran Indent
@subsubsection Syntactic Conventions

  Fortran mode assumes that you follow certain conventions that simplify
the task of understanding a Fortran program well enough to indent it
properly:

@vindex fortran-continuation-char
@itemize @bullet
@item
Two nested @samp{do} loops never share a @samp{continue} statement.

@item
The same character appears in column 5 of all continuation lines.  It
is the value of the variable @code{fortran-continuation-char}.
By default, this character is @samp{$}.
@end itemize

@noindent
If you fail to follow these conventions, the indentation commands may
indent some lines unaesthetically.  However, a correct Fortran program will
retain its meaning when reindented even if the conventions are not
followed.

@node ForIndent Vars,  , ForIndent Conv, Fortran Indent
@subsubsection Variables for Fortran Indentation

@vindex fortran-do-indent
@vindex fortran-if-indent
@vindex fortran-continuation-indent
@vindex fortran-check-all-num-for-matching-do
@vindex fortran-minimum-statement-indent
  Several additional variables control how Fortran indentation works.

@table @code
@item fortran-do-indent
Extra indentation within each level of @samp{do} statement (the default is 3).

@item fortran-if-indent
Extra indentation within each level of @samp{if} statement (the default is 3).

@item fortran-continuation-indent
Extra indentation for bodies of continuation lines (the default is 5).

@item fortran-check-all-num-for-matching-do
If this is @code{nil}, indentation assumes that each @samp{do}
statement ends on a @samp{continue} statement.  Therefore, when
computing indentation for a statement other than @samp{continue}, it
can save time by not checking for a @samp{do} statement ending there.
If this is non-@code{nil}, indenting any numbered statement must check
for a @samp{do} that ends there.  The default is @code{nil}.

@item fortran-minimum-statement-indent
Minimum indentation for Fortran statements.  For standard Fortran,
this is 6.  Statement bodies are always indented at least this much.
@end table

@node Fortran Comments, Fortran Columns, Fortran Indent, Fortran
@subsection Comments

  The usual Emacs comment commands assume that a comment can follow a line
of code.  In Fortran, the standard comment syntax requires an entire line
to be just a comment.  Therefore, Fortran mode replaces the standard Emacs
comment commands and defines some new variables.

  Fortran mode can also handle a non-standard comment syntax where comments
start with @samp{!} and can follow other text.  Because only some Fortran
compilers accept this syntax, Fortran mode will not insert such comments
unless you have specified to do so in advance by setting the variable
@code{comment-start} to @samp{"!"} (@pxref{Variables}).

@table @kbd
@item M-;
Align comment or insert new comment (@code{fortran-comment-indent}).

@item C-x ;
Applies to nonstandard @samp{!} comments only.

@item C-c ;
Turn all lines of the region into comments, or (with arg)
turn them back into real code (@code{fortran-comment-region}).
@end table

  @kbd{M-;} in Fortran mode is redefined as the command
@code{fortran-comment-indent}.  Like the usual @kbd{M-;} command,
it recognizes an existing comment and aligns its text appropriately.
If there is no existing comment, a comment is inserted and aligned.

Inserting and aligning comments is not the same in Fortran mode as in
other modes.  When a new comment must be inserted, a full-line comment is
inserted if the current line is blank.  On a non-blank line, a
non-standard @samp{!} comment is inserted if you previously specified
you wanted to use them.  Otherwise a full-line comment is inserted on a
new line before the current line.

  Non-standard @samp{!} comments are aligned like comments in other
languages, but full-line comments are aligned differently.  In a
standard full-line comment, the comment delimiter itself must always
appear in column zero.  What can be aligned is the text within the
comment.  You can choose from three styles of alignment by setting the
variable @code{fortran-comment-indent-style} to one of these values:

@vindex fortran-comment-indent-style
@vindex fortran-comment-line-column
@table @code
@item fixed
The text is aligned at a fixed column, which is the value of
@code{fortran-comment-line-column}.  This is the default.
@item relative
The text is aligned as if it were a line of code, but with an
additional @code{fortran-comment-line-column} columns of indentation.
@item nil
Text in full-line columns is not moved automatically.
@end table

@vindex fortran-comment-indent-char
  You can also specify the character to be used to indent within
full-line comments by setting the variable @code{fortran-comment-indent-char}
to the character you want to use.

@vindex comment-line-start
@vindex comment-line-start-skip
  Fortran mode introduces two variables @code{comment-line-start} and
@code{comment-line-start-skip}, which do for full-line comments what
@code{comment-start} and @code{comment-start-skip} do for
ordinary text-following comments.  Normally these are set properly by
Fortran mode, so you do not need to change them.

  The normal Emacs comment command @kbd{C-x ;} has not been redefined.
It can therefore be used if you use @samp{!} comments, but is useless in
Fortran mode otherwise. 

@kindex C-c ; (Fortran mode)
@findex fortran-comment-region
@vindex fortran-comment-region
  The command @kbd{C-c ;} (@code{fortran-comment-region}) turns all the
lines of the region into comments by inserting the string @samp{C$$$} at
the front of each one.  With a numeric arg, the region is turned back into
live code by deleting @samp{C$$$} from the front of each line.  You can
control the string used for the comments by setting the variable
@code{fortran-comment-region}.  Note that here we have an example of a
command and a variable with the same name; the two uses of the name never
conflict because in Lisp and in Emacs it is always clear from the context
which one is referred to.

@node Fortran Columns, Fortran Abbrev, Fortran Comments, Fortran
@subsection Columns

@table @kbd
@item C-c C-r
Displays a ``column ruler'' momentarily above the current line
(@code{fortran-column-ruler}).
@item C-c C-w
Splits the current window horizontally so that it is 72 columns wide.
This may help you avoid going over that limit (@code{fortran-window-create}).
@end table

@kindex C-c C-r (Fortran mode)
@findex fortran-column-ruler
  The command @kbd{C-c C-r} (@code{fortran-column-ruler}) shows a column
ruler above the current line.  The comment ruler consists of two lines
of text that show you the locations of columns with special significance
in Fortran programs.  Square brackets show the limits of the columns for
line numbers, and curly brackets show the limits of the columns for the
statement body.  Column numbers appear above them.

  Note that the column numbers count from zero, as always in XEmacs.  As
a result, the numbers may not be those you are familiar with; but the
actual positions in the line are standard Fortran.

  The text used to display the column ruler is the value of the variable
@code{fortran-comment-ruler}.  By changing this variable, you can change
the display.

@kindex C-c C-w (Fortran mode)
@findex fortran-window-create
  For even more help, use @kbd{C-c C-w} (@code{fortran-window-create}), a
command which splits the current window horizontally, resulting in a window 72
columns wide.  When you edit in this window, you can immediately see
when a line gets too wide to be correct Fortran.

@node Fortran Abbrev,  , Fortran Columns, Fortran
@subsection Fortran Keyword Abbrevs

  Fortran mode provides many built-in abbrevs for common keywords and
declarations.  These are the same sort of abbrevs that you can define
yourself.  To use them, you must turn on Abbrev mode.  @pxref{Abbrevs}.

  The built-in abbrevs are unusual in one way: they all start with a
semicolon.  You cannot normally use semicolon in an abbrev, but Fortran
mode makes this possible by changing the syntax of semicolon to ``word
constituent''.

  For example, one built-in Fortran abbrev is @samp{;c} for
@samp{continue}.  If you insert @samp{;c} and then insert a punctuation
character such as a space or a newline, the @samp{;c} changes
automatically to @samp{continue}, provided Abbrev mode is enabled.@refill

  Type @samp{;?} or @samp{;C-h} to display a list of all built-in
Fortran abbrevs and what they stand for.

@node Asm Mode,  , Fortran, Programs
@section Asm Mode

@cindex Asm mode
Asm mode is a major mode for editing files of assembler code.  It
defines these commands:

@table @kbd
@item @key{TAB}
@code{tab-to-tab-stop}.
@item @key{LFD}
Insert a newline and then indent using @code{tab-to-tab-stop}.
@item :
Insert a colon and then remove the indentation from before the label
preceding colon.  Then do @code{tab-to-tab-stop}.
@item ;
Insert or align a comment.
@end table

  The variable @code{asm-comment-char} specifies which character
starts comments in assembler syntax.