function enqueue(op, force) {
    const q = op.stamp < this._clock;
    if (q) op.stamp = this._clock;
    if (q || force) {
      op.qrank = op.rank;
      this._heap.push(op);
    }
  }
Enqueue an operator into the priority queue for evaluation. The operator
will be enqueued if it has no registered pulse for the current cycle, or if
the force argument is true. Upon enqueue, this method also sets the
operator's qrank to the current rank value.
@param {Operator} op - The operator to enqueue.
@param {boolean} [force] - A flag indicating if the operator should be
  forceably added to the queue, even if it has already been previously
  enqueued during the current pulse propagation. This is useful when the
  dataflow graph is dynamically modified and the operator rank changes.