human
Solve the following python programming problem given the following code:
def callback_middleware(app) : 
    'Calls the request-endcallback that the app may have set

    Args:
        app: The WSGI app to wrap.

    Returns:
        The wrapped app, also a WSGI app.
    ' 
    def callback_wrapper(environ, start_response) : 
        'Call the WSGI app and then invoke the request-end callbacks.' 
        try : 
            return app (environ, start_response) 
finally : 
            callback.InvokeCallbacks () 
return <MASK> 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def callback_wrapper(