@classmethod
def at(cls, client, base_url=None):
    """
        Returns an object of type `cls` (must be a subclass of Client)
        at the same base URL as `client` or at the given `base_url`.
        Copies the session (and thus the authentication details) of `client`.
        """
    if base_url is None:
        base_url = client.base_url
    return cls(server=client.server, base_url=base_url, session=client.session)
