human
Solve the following python programming problem given the following code:
def download(self, download_func, request, spider) : 
    def process_request(request) : 
        for method in self.methods ['process_request'] : 
            response = method (request = request, spider = spider) 
            assert ((response is None) or isinstance (response, (Response, Request))), ('Middleware %s.process_request must return None, Response or Request, got %s' % (method.im_self.__class__.__name__, response.__class__.__name__)) 
            if response : 
                return response 
return download_func (request = request, spider = spider) 
def process_response(response) : 
        assert (response is not None), 'Received None in process_response' 
        if isinstance (response, Request) : 
            return response 
for method in self.methods ['process_response'] : 
            response = method (request = request, response = response, spider = spider) 
            assert isinstance (response, (Response, Request)), ('Middleware %s.process_response must return Response or Request, got %s' % (method.im_self.__class__.__name__, type (response))) 
            if isinstance (response, Request) : 
                return response 
return response 
def process_exception(_failure) : 
        exception = _failure.value 
        for method in self.methods ['process_exception'] : 
            response = method (request = request, exception = exception, spider = spider) 
            assert ((response is None) or isinstance (response, (Response, Request))), ('Middleware %s.process_exception must return None, Response or Request, got %s' % (method.im_self.__class__.__name__, type (response))) 
            if response : 
                return response 
return _failure 
deferred = mustbe_deferred (process_request, request) 
    deferred.addErrback (process_exception) 
    deferred.addCallback (<MASK>) 
    return deferred 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def process_response(