<expression>:
    ‘(’<expression>‘)’
    <expression> <binary operator> <expression>
    ‘+’ <expression>
    ‘-’ <expression>
    <left value>
    <left value> <assignment operator> <expression>
    <left value> ‘++’
    <left value> ‘--’
    ‘++’ <left value>
    ‘--’ <left value>
    ‘!’ <expression>
    <expression> ‘?’ <expression> ‘:’ <expression>
    <function> ‘(’ <expr list> ‘)’
    <function> ‘(’ <expr list> ‘|’ <option list> ‘)’
    <string>
    <exponent vector>
    <atom>
    <list>

<left value>:
    <program variable> [‘[’<expression>‘]’]*

<binary operator>:
    ‘+’ ‘-’ ‘*’ ‘/’ ‘%’ ‘^’(exponentiation)
    ‘==’ ‘!=’ ‘<’ ‘>’ ‘<=’ ‘>=’ ‘&&’ ‘||’
    ‘==’ ‘!=’ ‘<’ ‘>’ ‘<=’ ‘>=’ ‘&&’ ‘||’

<assignment operator>:
    ‘=’ ‘+=’ ‘-=’ ‘*=’ ‘/=’ ‘%=’ ‘^=’ 

<expr list>:
    <empty>
    <expression> [‘,’ <expression>]*

<option>:
    Character sequence beginning with an alphabetical letter ‘=’ <expr>

<option list>:
    <option>
    <option> [‘,’ <option>]*

<list>:
    ‘[’ <expr list> ‘]’

<program variable>:
   Sequence of alphabetical letters or numeric digits or _
   that begins with a capital alphabetical letter
   (X,Y,Japan etc.)

<function>:
   Sequence of alphabetical letters or numeric digits or _
   that begins with a small alphabetical letter
   (fctr,gcd etc.)

<atom>:
   <indeterminate>
   <number>

<indeterminate>:
   Sequence of alphabetical letters or numeric digits or _
   that begin with a small alphabetical letter
   (a,bCD,c1_2 etc.)

<number>:
   <rational number>
   <floating point number>
   <algebraic number>
   <complex number>

<rational number>:
   0, 1, -2, 3/4

<floating point number>:
   0.0, 1.2e10

<algebraic number>: 
   newalg(x^2+1), alg(0)^2+1

<complex number>:
   1+@i, 2.3*@i

<string>:
   character sequence enclosed by two ‘"’'s.

<exponent vector>:
   ‘<<’ <expr list> ‘>>’

<statement>:
    <expression> <terminator>
    <compound statement>
    ‘break’ <terminator>
    ‘continue’ <terminator>
    ‘return’ <terminator>
    ‘return’ <expression> <terminator>
    ‘if’ ‘(’ <expr list> ‘)’ <statement>
    ‘if’ ‘(’ <expr list> ‘)’ <statement> ‘else’ <statement>
    ‘for’ ‘(’ <expr list> ‘;’ <expr list> ‘;’ <expr list> ‘)’ <statement>
    ‘do’ <statement> ‘while’ ‘(’ <expr list> ‘)’ <terminator>
    ‘while’ ‘(’ <expr list> ‘)’ <statement>
    ‘def’ <function> ‘(’ <expr list> ‘)’ ‘{’ <variable declaration> <stat list> ‘}’
    ‘end(quit)’ <terminator>

<terminator>:
    ‘;’ ‘$’

<variable declaration>:
    [‘extern’ <program variable> [‘,’ <program variable>]* <terminator>]*

<compound statement>:
    ‘{’ <stat list> ‘}’

<stat list>:
    [<statement>]*