src/HOL/Tools/watcher.ML

 	       decr_watched())
 	 else (report i "System error in proof handler";
 	       decr_watched())
        end
  in Output.debug (fn () => ("subgoals forked to createWatcher: "^ prems_string_of th));
-    IsaSignal.signal (IsaSignal.usr2, IsaSignal.SIG_HANDLE proofHandler);
+    IsaSignal.signal (IsaSignal.usr1, IsaSignal.SIG_HANDLE proofHandler);
     (childin, childout, childpid)
   end
 
 end (* structure Watcher *)