code: func (h *Heap) addIfNotPresentLocked(key string, obj interface{}) {
  	if _, exists := h.data.items[key]; exists {
  		return
  	}
  	heap.Push(h.data, &itemKeyValue{key, obj})
  }
docs: // addIfNotPresentLocked assumes the lock is already held and adds the provided
  // item to the queue if it does not already exist.