<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def on(self, name, callback, once=False):
 """ Adds a callback to the event specified by name once <bool> if True the callback will be removed once it's been triggered """

        if name not in self.event_listeners:
            self.event_listeners[name] = []

        self.event_listeners[name].append((callback, once))