code: void flip(long index) {
  		assert index >= 0 && index < numBits : "index=" + index + " numBits=" + numBits;
  		int wordNum = (int) (index >> 6); // div 64
  		long bitmask = 1L << index; // mod 64 is implicit
  		bits[wordNum] ^= bitmask;
  	}
docs: Flip the bit at the provided index.