anchor.diff:    int count([bool expected = true]) {
       var tally = 0;
       for (var index = 0; index < length; index++) {
  -      var actual = (buffer[index >> _shift] & _setMask[index & _offset]) != 0;
  +      var actual = (buffer[index >> SHIFT] & SET_MASK[index & OFFSET]) != 0;
         if (actual == expected) {
           tally++;
         }
  
positive.code:   /// current value.
    void flip(int index) {
      if (0 <= index && index < length) {
        buffer[index >> _shift] ^= _setMask[index & _offset];
      } else {
        throw new RangeError.range(index, 0, length);
      }
  
negative.code:   void operator []=(int index, bool value) {
      if (0 <= index && index < length) {
        if (value) {
          buffer[index >> SHIFT] |= SET_MASK[index & OFFSET];
        } else {
          buffer[index >> SHIFT] &= CLEAR_MASK[index & OFFSET];
        }
      } else {
        throw new RangeError.range(index, 0, length);
  