code: def get_plaintext_to_sign(self):
          
          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)
docs: 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)