docs: @Override
    public void add(int value) {
      int key = toIntUnsigned(highbits(value));
      if (key != currentKey) {
        if (key < currentKey) {
          underlying.add(value);
          return;
        } else {
          appendToUnderlying();
          currentKey = key;
        }
      }
      C tmp = container.add(lowbits(value));
      if (tmp != container) {
        container = tmp;
      }
    }