code: def keypair(self, i, keypair_class):
          
  
          # Make sure keypair_class is a valid cryptocurrency keypair
          if not is_cryptocurrency_keypair_class(keypair_class):
              raise Exception(_messages["INVALID_KEYPAIR_CLASS"])
  
          currency_name = keypair_class.__name__.lower().replace('keypair', '')
  
          k = keypair_class.from_passphrase(
              self._passphrase + " " + currency_name + str(i))
  
          return k
docs: Return the keypair that corresponds to the provided sequence number
              and keypair class (BitcoinKeypair, etc.).