code: def subscribe(self, subject, callback, queue=''):
          
          s = Subscription(
              sid=self._next_sid,
              subject=subject,
              queue=queue,
              callback=callback,
              connetion=self
          )
  
          self._subscriptions[s.sid] = s
          self._send('SUB %s %s %d' % (s.subject, s.queue, s.sid))
          self._next_sid += 1
  
          return s
docs: Subscribe will express interest in the given subject. The subject can
          have wildcards (partial:*, full:>). Messages will be delivered to the
          associated callback.
  
          Args:
              subject (string): a string with the subject
              callback (function): callback to be called