code: function add(x, y) {
      var lsb = (x & 0xffff) + (y & 0xffff);
      var msb = (x >>>   16) + (y >>>   16) + (lsb >>> 16);
      return (msb << 16) | (lsb & 0xffff);
    }
docs: generation counter /* private: 32-bit integer safe adder