code: def delay_service(self, t=None):
          
          if len(self._departures) > 1:
              agent = heappop(self._departures)
  
              if t is None:
                  agent._time = self.service_f(agent._time)
              else:
                  agent._time = t
  
              heappush(self._departures, agent)
              self._update_time()
docs: Adds an extra service time to the next departing
          :class:`Agent's<.Agent>` service time.
  
          Parameters
          ----------
          t : float (optional)
              Specifies the departing time for the agent scheduled
              to depart next. If ``t`` is not given, then an additional
              service time is added to the next departing agent.