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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_keyup_callback(self, key, fn):
 """ Allows for custom callback functions for the viewer. Called on key up. Parameter 'any' will ensure that the callback is called on any key up, and block default mujoco viewer callbacks from executing, except for the ESC callback to close the viewer. """

        self.viewer.keyup[key].append(fn)