EXPR =
  '<expr>' [ DEFUN | DEFVAR | TERM ] * '</expr>'

TERM = [ INT | STRING | SYMBOL | LIST | VAR | FUNCALL ]

DEFUN =
  '<defun fname="' FNAME ? ">'
  ARGS ?
  TERM *
  '</defun>'

ARGS =
  '<args>' FIXED ? OPTIONAL ? REST ? '</args>'
  | '<args-unevalled>' FIXED ? OPTIONAL ? REST ? '</args-unevalled>'

FIXED =
  '<fixed> VAR + '</fixed>'

OPTIONAL =
  '<optional>' VAR + '<optional>'

REST =
  '<rest>' VAR '</rest>'

DEFVAR = DEFVAR-INT DEFVAR-STR

DEFVAR-INT =
  '<defvar vname="' VNAME '">'
  DESCRIPTION ?
  INT ?
  <possible-value> [ INT | '<range>' INT INT '</range>' ] *</possible-value>
  '</defvar>'

DEFVAR-STR =
  '<defvar vname"' VNAME '">'
  DESCRIPTION ?
  STR ?
  <possible-value> STR * </possible-value>
  '</defvar>'

DESCRIPTION =
  '<description>' [ TEXT | GETTEXT ] '</description>'

GETTEXT =
  '<gettext>' TEXT '</gettext>'

INT =
  '<integer>' INTEGER '</integer>'
STR =
  '<string>' STRING '</string>'
SYMBOL =
  '<symbol>' NAME '</symbol>'
LIST =
  '<list>' TERM * '</list>' | <list/>

VAR =
  '<varref vname="' VNAME '"/>'

FUNCALL =
  '<funcall fname="' FNAME '">' TERM * '</funcall>'
  | '<' PREDEFINED '>' TERM * '</' PREDEFINED '>'
  | '<' PREDEFINED 'vname="' VNAME '">' TERM * '</' PREDEFINED '>'

PREDEFINED =
  'set'
  | 'and' | 'or' | 'not'
  | 'lt' | 'le' | 'eq' | 'ge' | 'gt' | 'noteq'
  | 'add' | 'sub' | 'mul' | 'div' | 'mod'
  | 'logand' | 'logior' | 'logxor' | 'lsh'
  | 'append' | 'concat' | 'nth'  | 'copy' | 'ins' | 'del'
  | 'progn' | 'if' | 'when' | 'cond'
  | 'loop' | 'break' | 'while' | 'foreach'
  | 'quote' | 'eval' | 'type'

;; TERM has a value, and is evaluated to a TERM as below.
;; INT has an integer value, and is evaluated to itself .
;; STRING has a string value, and is evaluated to itself.
;; SYMBOL has a symbol value, and is evaluated to itself.
;; LIST has a list value and, is evaluated to itself.
;; VAR has a value of TERM that is assigned to the variable,
;;   and is evalued to that TERM.
;; FUNCALL has a value of TERM that is returned by that function,
;;   and is evalued to that TERM.

INTTERM = INT | VAR (value is INT) | FUNCALL (return INT)
STRTERM = STR | VAR (value is STR) | FUNCALL (return STR)
LISTTERM = LIST | VAR (value is LIST) | FUNCALL (return LIST)
ANYTERM = TERM

;; Here after, when an argument is TERM, it is given to a function
;; without being evaluted.  Otherwise, an evaluated term is given.

'<set vname="' VNAME '">' ANYTERM '</set>'
'<add>' INTTERM + '</add>'
'<add vname="' VNAME '">' INTTERM + '</add>'
'<sub>' INTTERM + '</sub>'
'<sub vname="' VNAME '">' INTTERM + '</sub>'
'<mul>' INTTERM + '</mul>'
'<mul vname="' VNAME '">' INTTERM + '</mul>'
'<div>' INTTERM + '</div>'
'<div vname="' VNAME '">' INTTERM + '</div>'
'<mod>' INTTERM INTTERM '</mod>'
'<mod vname="' VNAME '">' INTTERM '</div>'
'<logand> INTTERM + '</logand>'
'<logand vname="' VNAME '">' INTTERM + '</logand>'
'<logior> INTTERM + '</logior>'
'<logior> vname="' VNAME '">' INTTERM + '</logior>'
'<logxor> INTTERM + '</logxor>'
'<logxor> vname="' VNAME '">' INTTERM + '</logxor>'
'<lsh>' INTTERM INTTERM '</lsh>'
'<lsh vname="' VNAME '">' INTTERM '</mul>'
'<lt>' INTTERM INTTERM '</lt>'
'<le>' INTTERM INTTERM '</le>'
'<eq>' INTTERM INTTERM '</eq>'
'<noteq>' INTTERM INTTERM '</noteq>'
'<ge>' INTTERM INTTERM '</ge>'
'<gt>' INTTERM INTTERM '</gt>'
'<append>' ANYTERM + '</append>'
'<concat>' [ INTTERM | STRTERM | LISTTERM ] + '</concat>'
'<nth>' INTTERM [ STRTERM | LISTTERM ] '</nth>'
'<copy>' LISTTERM '</nth>'
'<ins vname="' VNAME '">' INTTERM [ STRTERM | LISTTERM ] '</ins>'
'<del vname="' VNAME '">' INTTERM INTTERM '</ins>'
'<and>' TERM + '</and>'
'<or>' TERM + '</or>'
'<not>' TERM '</not>'
'<progn>' TERM * '</progn>'
'<if>' TERM TERM TERM ? '</if>'
'<when>' TERM TERM * '</when>'
'<cond>' LIST + '</cond>'
'<loop>' TERM + '</loop>'
'<while>' TERM + '</while>'
'<foreach vname="' VNAME '">' TERM + '</foreach>'
'<quote>' TERM '</quote>'
'<eval>' TERM '</eval>'
'<type>' ANYTERM '</type>'