code: def request(self, subject, callback, msg=None):
          
          inbox = self._build_inbox()
          s = self.subscribe(inbox, callback)
          self.unsubscribe(s, 1)
          self.publish(subject, msg, inbox)
  
          return s
docs: ublish a message with an implicit inbox listener as the reply.
          Message is optional.
  
          Args:
              subject (string): a string with the subject
              callback (function): callback to be called
              msg (string=None): payload string