letter = "A" | "B" | "C" | "D" | "E" | "F" | "G"
       | "H" | "I" | "J" | "K" | "L" | "M" | "N"
       | "O" | "P" | "Q" | "R" | "S" | "T" | "U"
       | "V" | "W" | "X" | "Y" | "Z" ;
digit = "0" | "1" | "2" | "3" | "4" | "5" | "6" | "7" | "8" | "9" ;
symbol = "[" | "]" | "{" | "}" | "(" | ")" | "<" | ">"
       | "'" | '"' | "=" | "|" | "." | "," | ";" ;
character = letter | digit | symbol | " " ;

identifier = letter , { letter | digit | " " } ;
terminal = "'" , character , { character } , "'"
         | '"' , character , { character } , '"' ;

special = "?" , any , "?" ;

comment = (* this is a comment "" *) "(*" , any-symbol , "*)" ;
any-symbol = ? any visible character ? ; (* ? ... ? *)

lhs = identifier ;
rhs = identifier
     | terminal
     | comment , rhs
     | rhs , comment
     | "[" , rhs , "]"
     | "{" , rhs , "}"
     | "(" , rhs , ")"
     | rhs , "|" , rhs
     | rhs , "," , rhs ;

rule = lhs , "=" , rhs , ";" | comment ;
grammar = { rule } ;