fix multiple main thread actors not working

scripts/engine.cm

@@ -820,10 +820,13 @@ var prog_script = `(function ${cell.args.program.name()}_start($_, arg) { var ar
 
 var startfn = js.eval(cell.args.program, prog_script);
 $_.clock(_ => {
+  log.console(`actor %{cell.id} is now running its program.`)
   var val = startfn($_, cell.args.arg);
   
   if (val)
     throw new Error('Program must not return anything');
 })
 
+log.console("end. set a clock.");
+
 })()