在编辑、撤销和重做的过程中，模型是唯一保持不变的。

During the course of editing, undo, and redo, the model is the only thing that endures.

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这个魔法的中心是撤销和重做的能力。

At the heart of this magic is the ability to undo and redo.

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命令可用来验证用户的交互，并且提供撤销和重做支持。

Commands can be used to validate the user's interaction, and to provide undo and redo support.

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