docs: def restart_agent(self, agent_id, **kwargs):
          '''tells the host agent running in this agency to restart the agent.'''
          host_medium = self.get_medium('host_agent')
          agent = host_medium.get_agent()
          d = host_medium.get_document(agent_id)
          # This is done like this on purpose, we want to ensure that document
          # exists before passing it to the agent (even though he would handle
          # this himself).
          d.addCallback(
              lambda desc: agent.start_agent(desc.doc_id, **kwargs))
          return d