human
Solve the following python programming problem given the following code:
def _add_headers_cb(self, _result) : 
    d = defer.succeed (None) 
    def debug_cb(result, msg) : 
        log.debug (msg) 
        return result 
def header_callback_maker(type_) : 
        def header_callback(ts, flags, data) : 
            return self.on_header_data_added (type_, ts, flags, data) 
return header_callback 
def do_read_headers(_result, s, type_) : 
        return s.read_headers (header_callback_maker (type_)) 
log.debug ('[adding headers]') 
    for (s, type_) in d._tracks : 
        d.addCallback (do_read_headers, s, type_) 
d.addCallback (debug_cb, '[headers done..]') 
    return d 

    
    

    Fix the buggy line: for (s, type_) in d._tracks :
assistant
#Here is Python code to fix the buggy line: for (s, type_) in d.

for (s, type_) in self._tracks :