code: @Override
    public void flip(int e)
    {
      int wordIndex = wordIndex(e);
      expandTo(wordIndex);
      int mask = (1 << e);
      words[wordIndex] ^= mask;
      fixFirstEmptyWord();
      if (size >= 0) {
        if ((words[wordIndex] & mask) == 0) {
          size--;
        } else {
          size++;
        }
      }
    }
docs: {@inheritDoc}