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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_client(self, client):
 """ Adds the specified client to this manager. :param client: The client to add into this manager. :type client: :class:`revision.client.Client` :return: The ClientManager instance (method chaining) :rtype: :class:`revision.client_manager.ClientManager` """

        if not isinstance(client, Client):
            raise InvalidArgType()

        if self.has_client(client.key):
            return self

        self[client.key] = client

        return self