func AddOrigin(s, origin string) string
AddOrigin adds origin to s if s is not already a FQDN. Note that the result may not be a FQDN. If origin does not end with a ".", the result won't either. This implements the zonefile convention (specified in RFC 1035, Section "5.1. Format") that "@" represents the apex (bare) domain. i.e. AddOrigin("@", "foo.com.") returns "foo.com.".
func TrimDomainName(s, origin string) string
TrimDomainName trims origin from s if s is a subdomain. This function will never return "", but returns "@" instead (@ represents the apex domain).