src/Pure/Isar/outer_lex.ML

   (case try Symbol.explode str of
     SOME [s] => Symbol.is_symbolic s orelse is_sym_char s
   | SOME ss => forall is_sym_char ss
   | _ => false);
 
-fun is_sid s = s = ":" orelse is_symid s orelse Syntax.is_identifier s;
+fun is_sid "begin" = false
+  | is_sid ":" = true
+  | is_sid s = is_symid s orelse Syntax.is_identifier s;
 
 
 (* scan strings *)
 
 local