docs: protected String checkHeap() {
      for(int i = 1; i < size; i++) {
        final int parent = (i - 1) >>> 1;
        if(comparator.compare(queue[parent], queue[i]) > 0) {
          return "@" + parent + ": " + queue[parent] + " < @" + i + ": " + queue[i];
        }
      }
      return null;
    }