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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 add(self, private_key):
 """Add a key to the keyring. Args: private_key (PaillierPrivateKey):
 a key to add to this keyring. """

        if not isinstance(private_key, PaillierPrivateKey):
            raise TypeError("private_key should be of type PaillierPrivateKey, "
                            "not %s" % type(private_key))
        self.__keyring[private_key.public_key] = private_key