docs: public boolean checkedAdd(final int x) {
      final short hb = Util.highbits(x);
      final int i = highLowContainer.getIndex(hb);
      if (i >= 0) {
        Container c = highLowContainer.getContainerAtIndex(i);
        int oldCard = c.getCardinality();
        // we need to keep the newContainer if a switch between containers type
        // occur, in order to get the new cardinality
        Container newCont = c.add(Util.lowbits(x));
        highLowContainer.setContainerAtIndex(i, newCont);
        if (newCont.getCardinality() > oldCard) {
          return true;
        }
      } else {
        final ArrayContainer newac = new ArrayContainer();
        highLowContainer.insertNewKeyValueAt(-i - 1, hb, newac.add(Util.lowbits(x)));
        return true;
      }
      return false;
    }