code: def pop(self, n=None):
          
          if self._popped:
              assert n is None
              return []
          self._popped = True
  
          envs = []
          for i, instance in enumerate(self.instances):
              env = remote.Remote(
                  handle=self._handles[i],
                  vnc_address='{}:{}'.format(instance.host, instance.vnc_port),
                  vnc_password='openai',
                  rewarder_address='{}:{}'.format(instance.host, instance.rewarder_port),
                  rewarder_password='openai',
              )
              envs.append(env)
          return envs
docs: Call from main thread. Returns the list of newly-available (handle, env) pairs.