code: void xor(LongBitSet other) {
  		assert other.numWords <= numWords : "numWords=" + numWords + ", other.numWords=" + other.numWords;
  		int pos = Math.min(numWords, other.numWords);
  		while (--pos >= 0) {
  			bits[pos] ^= other.bits[pos];
  		}
  	}
docs: this = this XOR other