def mount(self, url, app):
        "Mount a sub-app at the url of current app."
        # Inspired by Bottle. It might seem that dispatching to
        # subapps would rather be handled by normal routes, but
        # arguably, that's less efficient. Taking into account
        # that paradigmatically there's difference between handing
        # an action and delegating responisibilities to another
        # app, Bottle's way was followed.
        app.url = url
        self.mounts.append(app)
Mount a sub-app at the url of current app.