docs: def list_agents(self):
          '''List agents hosted by the agency.'''
          t = text_helper.Table(fields=("Agent ID", "Agent class", "State"),
                                lengths=(40, 25, 15))
  
          return t.render((a._descriptor.doc_id, a.log_category,
                           a._get_machine_state().name)
                          for a in self._agents)