human
Solve the following python programming problem given the following code:
def add_callbacks(future, callback, errback) : 
    '
    callback or errback may be None, but at least one must be
    non-None.

    XXX beware the "f._result" hack to get "chainable-callback" type
    behavior.
    ' 
    def done(f) : 
        try : 
            res = f.result () 
            if callback : 
                x = callback (res) 
                if (x is not None) : 
                    f._result = x 
except Exception : 
            if errback : 
                errback (create_failure ()) 
return future.add_done_callback (errback) 

    
    

    Fix the buggy line: return future.add_done_callback (errback)
assistant
#Here is Python program to fix the buggy line: return future.

return future.add_done_callback (def done()