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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 revoker(self, revoker, **prefs):
 """ Generate a signature that specifies another key as being valid for revoking this key. :param revoker: The :py:obj:`PGPKey` to specify as a valid revocation key. :type revoker: :py:obj:`PGPKey` :raises: :py:exc:`~pgpy.errors.PGPError` if the key is passphrase-protected and has not been unlocked :raises: :py:exc:`~pgpy.errors.PGPError` if the key is public :returns: :py:obj:`PGPSignature` In addition to the optional keyword arguments accepted by :py:meth:`PGPKey.sign`, the following optional keyword arguments can be used with :py:meth:`PGPKey.revoker`. :keyword sensitive: If ``True``, this sets the sensitive flag on the RevocationKey subpacket. Currently, this has no other effect. :type sensitive: ``bool`` """

        hash_algo = prefs.pop('hash', None)

        sig = PGPSignature.new(SignatureType.DirectlyOnKey, self.key_algorithm, hash_algo, self.fingerprint.keyid)

        # signature options that only make sense when adding a revocation key
        sensitive = prefs.pop('sensitive', False)
        keyclass = RevocationKeyClass.Normal | (RevocationKeyClass.Sensitive if sensitive else 0x00)

        sig._signature.subpackets.addnew('RevocationKey',
                                         hashed=True,
                                         algorithm=revoker.key_algorithm,
                                         fingerprint=revoker.fingerprint,
                                         keyclass=keyclass)

        # revocation keys should really not be revocable themselves
        prefs['revocable'] = False
        return self._sign(self, sig, **prefs)