code: def sign(self, data, nonce = None):
          
  
          cls = self.__class__
  
          if not self.__mont_priv:
              raise MissingKeyException(
                  "Cannot sign using this XEdDSA instance, Montgomery private key missing."
              )
  
          if not isinstance(data, bytes):
              raise TypeError("The data parameter must be a bytes-like object.")
  
          if nonce == None:
              nonce = os.urandom(64)
  
          if not isinstance(nonce, bytes):
              raise TypeError("Wrong type passed for the nonce parameter.")
  
          if len(nonce) != 64:
              raise ValueError("Invalid value passed for the nonce parameter.")
  
          ed_priv, ed_pub = cls._mont_priv_to_ed_pair(bytearray(self.__mont_priv))
  
          return bytes(cls._sign(
              bytearray(data),
              bytearray(nonce),
              ed_priv,
              ed_pub
          ))
docs: Sign data using the Montgomery private key stored by this XEdDSA instance.
  
          :param data: A bytes-like object containing the data to sign.
          :param nonce: A bytes-like object with length 64 or None.
          :returns: A bytes-like object encoding the signature with length SIGNATURE_SIZE.
  
          If the nonce parameter is None, a new nonce is generated and used.
  
          :raises MissingKeyException: If the Montgomery private key is not available.