code: @Override
    public boolean retainAll(IntSet c)
    {
      if (c == null || c.isEmpty()) {
        return false;
      }
      boolean res = false;
      for (int i = 0; i < cells.length; i++) {
        if (cells[i] >= 0 && !c.contains(cells[i])) {
          cells[i] = REMOVED;
          res = true;
          size--;
        }
      }
      if (res) {
        modCount++;
      }
      return res;
    }
docs: {@inheritDoc}