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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def add_event_handler(self, callback, event=None):
 """ Registers the given callback to be called on the specified event. Args: callback (`callable`):
 The callable function accepting one parameter to be used. Note that if you have used `telethon.events.register` in the callback, ``event`` will be ignored, and instead the events you previously registered will be used. event (`_EventBuilder` | `type`, optional):
 The event builder class or instance to be used, for instance ``events.NewMessage``. If left unspecified, `telethon.events.raw.Raw` (the :tl:`Update` objects with no further processing) will be passed instead. """

        builders = events._get_handlers(callback)
        if builders is not None:
            for event in builders:
                self._event_builders.append((event, callback))
            return

        if isinstance(event, type):
            event = event()
        elif not event:
            event = events.Raw()

        self._event_builders.append((event, callback))