code: func availableAddresses(allAddrs []net.IP) (v4 []net.IP, v6 []net.IP) {
  	for _, addr := range allAddrs {
  		if addr.To4() != nil {
  			v4 = append(v4, addr)
  		} else {
  			v6 = append(v6, addr)
  		}
  	}
  	return
  }
docs: // availableAddresses takes a ValidationRecord and splits the AddressesResolved
  // into a list of IPv4 and IPv6 addresses.