} else {
        index = crossOver;
        heap = otherHeap;
      }
      heap.bubbleUpAlternatingLevels(index, x);
    }

    /**
     * Bubbles a value from {@code index} up the levels of this heap, and returns the index the
     * element ended up at.
     */
    @CanIgnoreReturnValue
    int bubbleUpAlternatingLevels(int index, E x) {
      while (index > 2) {