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