for i := 0; i < len(z) && i < len(x); i++ {
		zi, cc := bits.Add(uint(x[i]), uint(c), 0)
		z[i] = Word(zi)
		c = Word(cc)
	}
	return
}

// addVWlarge is addVW, but intended for large z.
// The only difference is that we check on every iteration
// whether we are done with carries,
// and if so, switch to a much faster copy instead.
// This is only a good idea for large z,