model, US&R model, triadic model, and selective undo.

Some common properties of models are:

* stable execution property: A state is represented as an ordered list of commands. This means that a command "is always undone in the state that was reached after the original execution."
* weakened stable execution: This means that if undo is executed all commands which depend on the undone command are undone dependent on the command.
* stable result property: This property has the similar meaning like the stable execution

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