def __call__(request, elements, kw):
    """ A pregenerator is a function associated by a developer with a
        :term:`route`. The pregenerator for a route is called by
        :meth:`pyramid.request.Request.route_url` in order to adjust the set
        of arguments passed to it by the user for special purposes, such as
        Pylons 'subdomain' support.  It will influence the URL returned by
        ``route_url``.

        A pregenerator should return a two-tuple of ``(elements, kw)``
        after examining the originals passed to this function, which
        are the arguments ``(request, elements, kw)``.  The simplest
        pregenerator is::

            def pregenerator(request, elements, kw):
                return elements, kw

        You can employ a pregenerator by passing a ``pregenerator``
        argument to the
        :meth:`pyramid.config.Configurator.add_route`
        function.

        """
