Get back the plaintext that will be signed.
        It is derived from the serialized zone file strings,
        but encoded as a single string (omitting the signature field,
        if already given)

def get_plaintext_to_sign(self):
        """
        Get back the plaintext that will be signed.
        It is derived from the serialized zone file strings,
        but encoded as a single string (omitting the signature field,
        if already given)
        """
        as_strings = self.pack_subdomain()
        if self.sig is not None:
            # don't sign the signature
            as_strings = as_strings[:-1]

        return ",".join(as_strings)