func (ip Addr) Prev() Addr {
	if ip.Is4() {
		if uint32(ip.addr.lo) == 0 {
			return Addr{}
		}
	} else if ip.addr.isZero() {
		return Addr{}
	}
	ip.addr = ip.addr.subOne()
	return ip
}

// String returns the string form of the IP address ip.
// It returns one of 5 forms:
//
//   - "invalid IP", if ip is the zero [Addr]
//   - IPv4 dotted decimal ("192.0.2.1")