human
Solve the following python programming problem given the following code:
def __init__(self, io_loop = None) : 
    if (not io_loop) : 
        io_loop = tornado.ioloop.IOLoop.current () 
self._io_loop = io_loop 
    self._readers = { 
        
} 
    self._writers = { 
        
} 
    self._fds = { 
        
} 
    self._delayedCalls = { 
        
} 
    PosixReactorBase.__init__ (self) 
    self.addSystemEventTrigger ('during', 'shutdown', self.crash) 
    def start_if_necessary() : 
        if (not self._started) : 
            self.fireSystemEvent ('startup') 
self._io_loop.add_callback (<MASK>) 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def start_if_necessary(