human
Solve the following python programming problem given the following code:
def register(handlers = None, subdomain = None, parent_dir = None, urls = None) : 
    "
    Register additional handlers to the server.

    :param handlers: List of handler classes to register.
    :param subdomain: The desired subdomain
    :param parent_dir: The directory under which all the handlers should be
                       placed
    :param urls: List of lists like [['path', Handler]].
                 Overwrites handlers input.
    " 
    if (subdomain is None) : 
        subdomain = '[A-Za-z0-9.]*' 
else : 
        subdomain = '{}\\.[A-Za-z0-9.]*'.format (subdomain) 
if (parent_dir is None) : 
        parent_dir = '' 
else : 
        parent_dir = '/{}'.format (parent_dir) 
if (not urls) : 
        def doc_url(handler) : 
            " reads the url regexp from the handler's docstring. " 
            docs = handler.__doc__ 
            pieces = docs.strip ().split ('

') [0].split ('
') 
            return ''.join ([piece.strip () for piece in pieces]) 
handlers = [tornado.web.URLSpec ('{}{}'.format (parent_dir, doc_url (handler)), handler) for handler in handlers] 
else : 
        handlers = urls 
def add_handler_callback(server, subdmn, hndls) : 
        ' add handler to server ' 
        server.add_handlers (subdmn, hndls) 
ioloop = tornado.ioloop.IOLoop.instance () 
    ioloop.add_callback (subdomain, SERVER, subdomain, handlers) 

    
    

    Fix the buggy line: ioloop.add_callback (subdomain, SERVER, subdomain, handlers)
assistant
#Here is Python program to fix the buggy line: ioloop.

ioloop.add_callback (def add_handler_callback(, SERVER, subdomain, handlers)