added begin parser;

--- a/src/Pure/Isar/outer_parse.ML	Wed Oct 11 22:55:19 2006 +0200
+++ b/src/Pure/Isar/outer_parse.ML	Wed Oct 11 22:55:20 2006 +0200
@@ -39,6 +39,7 @@
   val tags: token list -> string list * token list
   val opt_unit: token list -> unit * token list
   val opt_keyword: string -> token list -> bool * token list
+  val begin: token list -> string * token list
   val opt_begin: token list -> bool * token list
   val nat: token list -> int * token list
   val enum: string -> (token list -> 'a * token list) -> token list -> 'a list * token list
@@ -191,7 +192,8 @@
 val opt_unit = Scan.optional ($$$ "(" -- $$$ ")" >> (K ())) ();
 fun opt_keyword s = Scan.optional ($$$ "(" |-- !!! (($$$ s >> K true) --| $$$ ")")) false;
 
-val opt_begin = Scan.optional ($$$ "begin" >> K true) false;
+val begin = $$$ "begin";
+val opt_begin = Scan.optional (begin >> K true) false;
 
 
 (* enumerations *)