code: def find_agent(self, desc):
          '''Gives medium class of the agent if the agency hosts it.'''
          agent_id = (desc.doc_id
                      if IDocument.providedBy(desc)
                      else desc)
          self.log("I'm trying to find the agent with id: %s", agent_id)
          result = first(x for x in self._agents
                         if x._descriptor.doc_id == agent_id)
          return defer.succeed(resultf find_agent(self, desc):
          '''Gives medium class of the agent if the agency hosts it.'''
          agent_id = (desc.doc_id
                      if IDocument.providedBy(desc)
                      else desc)
          self.log("I'm trying to find the agent with id: %s", agent_id)
          result = first(x for x in self._agents
                         if x._descriptor.doc_id == agent_id)
          return defer.succeed(result)
docs: Gives medium class of the agent if the agency hosts it.