def __call__(value, system):
    """ Call the renderer with the result of the
        view (``value``) passed in and return a result (a string or
        unicode object useful as a response body).  Values computed by
        the system are passed by the system in the ``system``
        parameter, which is a dictionary.  Keys in the dictionary
        include: ``view`` (the view callable that returned the value),
        ``renderer_name`` (the template name or simple name of the
        renderer), ``context`` (the context object passed to the
        view), and ``request`` (the request object passed to the
        view)."""
