human
Solve the following python programming problem given the following code:
def reduce_result(iter, **kwargs) : 
    import csv 
    import sys 
    def default_flush_callback(stream) : 
        stream.flush () 
        return stream 
output_stream = kwargs.get ('output_stream', sys.stdout) 
    flush_schedule = kwargs.get ('flush_schedule', 5000) 
    flush_callback = kwargs.get ('flush_callback', <MASK>) 
    writer = csv.writer (output_stream) 
    count = 0 
    for (keys, values) in iter : 
        data = (keys + values) 
        row = [unicode (x).encode ('utf-8') for x in data] 
        writer.writerow (row) 
        count += 1 
        if (count > flush_schedule) : 
            output_stream = flush_callback (output_stream) 
            count = 0 
flush_callback (output_stream) 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def default_flush_callback(