after  uint64
	}
	magic64 := uint64(magic64)
	x.before = magic64
	x.after = magic64
	x.i = 0xfffffffffffffff
	j := x.i
	for mask := uint64(1); mask != 0; mask <<= 1 {
		old := x.i
		k := AndUint64(&x.i, ^mask)
		j &= ^mask
		if x.i != j || k != old {
			t.Fatalf("mask=%d i=%d j=%d k=%d old=%d", mask, x.i, j, k, old)
		}
	}
	if x.before != magic64 || x.after != magic64 {