diff -r 1ce77362598f -r a902f158b4fc src/Pure/Isar/outer_syntax.ML
--- a/src/Pure/Isar/outer_syntax.ML	Tue Jul 20 14:41:13 2010 +0200
+++ b/src/Pure/Isar/outer_syntax.ML	Tue Jul 20 14:44:33 2010 +0200
@@ -62,7 +62,7 @@
   (case cmd name of
     SOME (Command {int_only, parse, ...}) =>
       Parse.!!! (Scan.prompt (name ^ "# ") (Parse.tags |-- parse >> pair int_only))
-  | NONE => sys_error ("no parser for outer syntax command " ^ quote name));
+  | NONE => raise Fail ("No parser for outer syntax command " ^ quote name));
 
 in