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-SYM

DEFVAR-INT =
  '<defvar vname="' VNAME '">'
  DESCRIPTION ?
  [ INT POSSIBLE-INT-VALUE ? ] ?
  '</defvar>'

POSSIBLE-INT-VALUE =
  <possible-value> [ INT | '<range>' INT INT '</range>' ] + </possible-value>

DEFVAR-STR =
  '<defvar vname"' VNAME '">'
  DESCRIPTION ?
  [ STR DEFVAR-STR_VALUE ? ] ?
  '</defvar>'

DEFVAR-STR_VALUE =
  <possible-value> STR + </possible-value>

DEFVAR-SYM =
  '<defvar vname"' VNAME '">'
  DESCRIPTION ?
  [ SYMBOL POSSIBLE-SYM-VALUE ? ] ?
  '</defvar>'

POSSIBLE-SYM-VALUE =
  <possible-value> SYMBOL + </possible-value>

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

PREDEFINED =
  SET
  | AND | OR | NOT
  | LT | LE | EQ | GE | GT | NOTEQ
  | ADD | SUB | MUL | DIV | MOD
  | LOGAND | LOGIOR | LOGXOR | LSH
  | APPEND | CONCAT | LENGTH | NTH  | COPY | INS | DEL
  | PROGN | IF | WHEN | COND
  | LOOP | WHILE | FOREACH | BREAK | RETURN
  | 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)
SYMTERM = SYMBOL | VAR (value is SYMBOL) | FUNCALL (return SYMBOL)
LISTTERM = LIST | VAR (value is LIST) | FUNCALL (return LIST)

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

SET = '<set vname="' VNAME '">' TERM '</set>'
ADD = '<add>' INTTERM + '</add>'
      | '<add vname="' VNAME '">' INTTERM + '</add>'
SUB = '<sub>' INTTERM + '</sub>'
      | '<sub vname="' VNAME '">' INTTERM + '</sub>'
MUL = '<mul>' INTTERM + '</mul>'
      | '<mul vname="' VNAME '">' INTTERM + '</mul>'
DIB = '<div>' INTTERM + '</div>'
      | '<div vname="' VNAME '">' INTTERM + '</div>'
MOD = '<mod>' INTTERM INTTERM '</mod>'
      | '<mod vname="' VNAME '">' INTTERM '</div>'
LOGAND = '<logand> INTTERM + '</logand>'
         | '<logand vname="' VNAME '">' INTTERM + '</logand>'
LOGIOR = '<logior> INTTERM + '</logior>'
         | '<logior vname="' VNAME '">' INTTERM + '</logior>'
LOGXOR = '<logxor> INTTERM + '</logxor>'
         | '<logxor vname="' VNAME '">' INTTERM + '</logxor>'
LSH = '<lsh>' INTTERM INTTERM '</lsh>'
      | '<lsh vname="' VNAME '">' INTTERM '</mul>'
LT = '<lt>' INTTERM INTTERM '</lt>'
LE = '<le>' INTTERM INTTERM '</le>'
EQ = '<eq>' INTTERM INTTERM '</eq>'
NOTEQ = '<noteq>' INTTERM INTTERM '</noteq>'
GE = '<ge>' INTTERM INTTERM '</ge>'
GT ='<gt>' INTTERM INTTERM '</gt>'
APPEND = '<append>' TERM + '</append>'
         | '<append vname=" VNAME '">' TERM + '</append>'
CONCAT = '<concat>' [ INTTERM | STRTERM | LISTTERM ] + '</concat>'
         | '<concat vname=" VNAME '">' [ INTTERM | STRTERM | LISTTERM ] + '</concat>'
LENGTH = '<length>' [ STRTERM | LISTTERM ] '</length>'
NTH = '<nth>' INTTERM [ STRTERM | LISTTERM ] '</nth>'
COPY = '<copy>' LISTTERM '</nth>'
INS = '<ins vname="' VNAME '">' INTTERM [ STRTERM | LISTTERM ] '</ins>'
DEL = '<del vname="' VNAME '">' INTTERM INTTERM '</ins>'
AND = '<and>' TERM + '</and>'
OR = '<or>' TERM + '</or>'
NOT = '<not>' TERM '</not>'
PROGN = '<progn>' TERM * '</progn>'
IF = '<if>' TERM TERM TERM ? '</if>'
WHEN = '<when>' TERM TERM * '</when>'
COND = '<cond>' LIST + '</cond>'
LOOP = '<loop>' TERM + '</loop>'
WHILE = '<while>' TERM + '</while>'
FOREACH = '<foreach vname="' VNAME '">' TERM + '</foreach>'
BREAK = '<break>' TERM ? '</break>'
RETURN = '<return>' TERM ? '</return>'
QUOTE = '<quote>' TERM '</quote>'
EVAL = '<eval>' TERM '</eval>'
TYPE = '<type>' TERM '</type>'