human
Solve the following python programming problem given the following code:
def before_send(self) : 
    def on_session_start(event) : 
        try : 
            self.xmpp.send_presence () 
            self.xmpp.get_roster () 
            self._session_started = True 
except self.lib.exceptions.XMPPError as e : 
            raise MessengerWarmupException (('XMPP Error: %s' % e)) 
self.xmpp = self.lib.ClientXMPP (self.from_jid, self.password) 
    self.xmpp.add_event_handler ('session_start', <MASK>) 
    result = self.xmpp.connect (address = (self.host, self.port), reattempt = False, use_tls = self.use_tls, use_ssl = self.use_ssl) 
    if result : 
        self.xmpp.process (block = False) 
    
    Given the code above, what is a proper replacement for <MASK>?
assistant
#Here is a proper replacement for <MASK>.
def on_session_start(