code: @Override
    public boolean contains(final int x) {
      final short hb = Util.highbits(x);
      final Container c = highLowContainer.getContainer(hb);
      return c != null && c.contains(Util.lowbits(x));
    }
docs: Checks whether the value is 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.