code: @Override
    public boolean contains(final int x) {
      final short hb = highbits(x);
      int index = highLowContainer.getContainerIndex(hb);
      return index >= 0
          && highLowContainer.containsForContainerAtIndex(index, lowbits(x));
    }
docs: Checks whether the value in included, which is equivalent to checking if the corresponding bit
  is set (get in BitSet class).
  
  @param x integer value
  @return whether the integer value is included.