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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 _hasher_first_run(self, preimage):
        '''
        Invoke the backend on-demand, and check an expected hash result,
        then replace this first run with the new hasher method.
        This is a bit of a hacky way to minimize overhead on hash calls after this first one.
        '''
        new_hasher = self._backend.keccak256
        assert new_hasher(b'') == b"\xc5\xd2F\x01\x86\xf7#<\x92~}\xb2\xdc\xc7\x03\xc0\xe5\x00\xb6S\xca\x82';\x7b\xfa\xd8\x04]\x85\xa4p"  # noqa: E501
        self.hasher = new_hasher
        return new_hasher(preimage)