docs: def lookup(self, aid):
          '''
          Gets a new proxy that references to the actor of this host
          (only actors in this host) identified by the given ID.
  
          This method can be called remotely synchronously.
  
          :param str. aid: identifier of the actor you want.
          :return: :class:`~.Proxy` of the actor requiered.
          :raises: :class:`NotFoundError`  if the actor does not exist.
          :raises: :class:`HostDownError`  if the host is down.
          '''
          if not self.alive:
              raise HostDownError()
          url = '%s://%s/%s' % (self.transport, self.host_url.netloc, aid)
          if url in self.actors.keys():
              return Proxy(self.actors[url])
          else:
              raise NotFoundError(url)