(* syntax-sig.sml * * COPYRIGHT (c) 1995 AT&T Bell Laboratories. * * This is the abstract syntax tree used to represent regular expressions. * It serves as the glue between different front-ends (implementing * different RE specification languages), and different back-ends (implementing * different compilation/searching algorithms). *)signature REGEXP_SYNTAX =  sig      exception CannotParse      exception CannotCompile      structure CharSet : ORD_SET where type Key.ord_key = char      datatype syntax        = Group of syntax	| Alt of syntax list	| Concat of syntax list	| Interval of (syntax * int * int option)	| Option of syntax	(* == Interval(re, 0, SOME 1) *)	| Star of syntax	(* == Interval(re, 0, NONE) *)	| Plus of syntax	(* == Interval(re, 1, NONE) *)	| MatchSet of CharSet.set	| NonmatchSet of CharSet.set	| Char of char	| Begin                   (* Matches beginning of stream *)	| End                     (* Matches end of stream *)      val addRange : CharSet.set * char * char -> CharSet.set      val allChars : CharSet.set	    end;