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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:
 def verify(self, keys=None):
 """ Verify that the assertion is syntactically correct and the signature is correct if present. :param keys: If not the default key file should be used then use one of these. """


        try:
            res = self._verify()
        except AssertionError as err:
            logger.error("Verification error on the response: %s", err)
            raise
        else:
            if res is None:
                return None

        if not isinstance(self.response, samlp.Response):
            return self

        if self.parse_assertion(keys):
            return self
        else:
            logger.error("Could not parse the assertion")
            return None