docs: def instance(cls, interval=5):
          '''
          Returns existing instance of messenger. If one does not exist it will
          be created and returned.
  
          :param int interval:
              Number of miliseconds that represents interval when messages will
              be processed.
              Note that this parameter will be used only the first time when
              instance is requested, every other time it will be ignored
              because existing instance of :class:`._Messenger` is returned.
          '''
          if not cls._instance:
              cls._instance = _Messenger(interval)
          return cls._instance