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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 async def _assert_link_secret(self, action: str) -> str: """ Return current wallet link secret label. Raise AbsentLinkSecret if link secret is not set. :param action: action requiring link secret """


        rv = await self.wallet.get_link_secret_label()
        if rv is None:
            LOGGER.debug('HolderProver._assert_link_secret: action %s requires link secret but it is not set', action)
            raise AbsentLinkSecret('Action {} requires link secret but it is not set'.format(action))

        return rv