<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sign(self, message: Message):
 """ Sign message inplace. """

        if not isinstance(message, SignedMessage):
            raise ValueError('{} is not signable.'.format(repr(message)))

        message.sign(self.signer)