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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 revoke(self, target, **prefs):
 """ Revoke a key, a subkey, or all current certification signatures of a User ID that were generated by this key so far. :param target: The key to revoke :type target: :py:obj:`PGPKey`, :py:obj:`PGPUID` :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.revoke`. :keyword reason: Defaults to :py:obj:`constants.RevocationReason.NotSpecified` :type reason: One of :py:obj:`constants.RevocationReason`. :keyword comment: Defaults to an empty string. :type comment: ``str`` """

        hash_algo = prefs.pop('hash', None)
        if isinstance(target, PGPUID):
            sig_type = SignatureType.CertRevocation

        elif isinstance(target, PGPKey):
            ##TODO: check to make sure that the key that is being revoked:
            #        - is this key
            #        - is one of this key's subkeys
            #        - specifies this key as its revocation key
            if target.is_primary:
                sig_type = SignatureType.KeyRevocation

            else:
                sig_type = SignatureType.SubkeyRevocation

        else:  # pragma: no cover
            raise TypeError

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

        # signature options that only make sense when revoking
        reason = prefs.pop('reason', RevocationReason.NotSpecified)
        comment = prefs.pop('comment', "")
        sig._signature.subpackets.addnew('ReasonForRevocation', hashed=True, code=reason, string=comment)

        return self._sign(target, sig, **prefs)