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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 if_mousedown(handler):
 """ Decorator for mouse handlers. Only handle event when the user pressed mouse down. (When applied to a token list. Scroll events will bubble up and are handled by the Window.) """

    def handle_if_mouse_down(mouse_event):
        if mouse_event.event_type == MouseEventType.MOUSE_DOWN:
            return handler(mouse_event)
        else:
            return NotImplemented
    return handle_if_mouse_down