docs: def find_agent(self, agent_id):
          """
          Return the medium class of the agent with agent_id if the one is
          running in simulation.
          """
          try:
              recp = IRecipient(agent_id)
              agent_id = recp.key
          except TypeError:
              pass
          agency = self.find_agency(agent_id)
          if agency:
              agent = yield agency.find_agent(agent_id)
              defer.returnValue(agent)