src/Pure/Isar/isar_syn.ML

   OuterSyntax.improper_command "Isabelle.command" "nested Isabelle command" K.control
     (P.position P.string :|-- (fn (str, pos) =>
       (case OuterSyntax.parse pos str of
         [transition] => Scan.succeed (K transition)
       | _ => Scan.fail_with (K "exactly one command expected"))
-      handle ERROR msg => Scan.fail_with (K ("malformed command\n" ^ msg))));
+      handle ERROR msg => Scan.fail_with (K msg)));
 
 
 
 (** diagnostic commands (for interactive mode only) **)