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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 on_change(self, *callbacks):
        ''' Provide callbacks to invoke if the document or any Model reachable
        from its roots changes.

        '''
        for callback in callbacks:

            if callback in self._callbacks: continue

            _check_callback(callback, ('event',))

            self._callbacks[callback] = callback