def __call__(environ, start_response):
    """Called by a WSGI server.

        The ``environ`` parameter is a dictionary object, containing CGI-style
        environment variables. This object must be a builtin Python dictionary
        (not a subclass, UserDict or other dictionary emulation), and the
        application is allowed to modify the dictionary in any way it
        desires. The dictionary must also include certain WSGI-required
        variables (described in a later section), and may also include
        server-specific extension variables, named according to a convention
        that will be described below.

        The ``start_response`` parameter is a callable accepting two required
        positional arguments, and one optional argument. For the sake of
        illustration, we have named these arguments ``status``,
        ``response_headers``, and ``exc_info``, but they are not required to
        have these names, and the application must invoke the
        ``start_response`` callable using positional arguments
        (e.g. ``start_response(status, response_headers)``).
        """
