human
Solve the following python programming problem given the following code:
def conditional_http_tween_factory(handler, registry) : 
    'A tween that sets up conditional response handling for some requests.' 
    def conditional_http_tween(request) : 
        response = handler (request) 
        if (response.last_modified is not None) : 
            response.conditional_response = True 
if (response.etag is not None) : 
            response.conditional_response = True 
            return response 
have_buffered_response = (isinstance (response.app_iter, collections.Sequence) and (len (response.app_iter) == 1)) 
        cacheable = ((request.method in { 'GET', 'HEAD' }) and (response.status_code == 200)) 
        if (have_buffered_response and cacheable) : 
            response.conditional_response = True 
            response.md5_etag () 
return response 
return <MASK> 
    
    Given the code above, what is a proper replacement for <MASK>? Choose among: handler, registry
assistant
#Here is a proper replacement for <MASK>.
def conditional_http_tween(