code: @Override
    public int indexOf(int e)
    {
      if (e < 0) {
        throw new IllegalArgumentException("positive integer expected: " + Integer.toString(e));
      }
      if (isEmpty()) {
        return -1;
      }
  
      int index = wordIndex(e);
      if (index >= firstEmptyWord || (words[index] & (1 << e)) == 0) {
        return -1;
      }
      int count = BitCount.count(words, index);
      count += Integer.bitCount(words[index] & ~(ALL_ONES_WORD << e));
      return count;
  
    }
docs: {@inheritDoc}