code: def enqueue(self, state):
          
          # save the state to secondary storage
          state_id = self._workspace.save_state(state)
          self.put(state_id)
          self._publish('did_enqueue_state', state_id, state)
          return state_id
docs: Enqueue state.
              Save state on storage, assigns an id to it, then add it to the
              priority queue