code: def subscribe_async(self, event, handler):
          
          self._handlers.async_handlers[event].append(handler)
docs: Subscribes an asynchronous Handler for the given Event.
  
          An asynchronous handler is executed concurrently to the others
          without blocking the Events flow.
  
          @param event: (str|see.Event) event to react to.
          @param handler: (callable) function or method to subscribe.