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Undo is the facility by which previous modifications to the buffer can be undone, returning the buffer state to what it was prior to some modification.
Undo is organized into a separate module. This module conceptually maintains a tree where the nodes represent application states and the arcs represent transitions between these states. The root of the tree represents the initial state of the application. The undo module also maintains a current state. During normal application operation, the current state is a leaf of a fairly long branch of the tree. Normal application operations add new nodes to the end of this branch. Moving the current state up the tree corresponds to an undo operation and moving it down some branch corresponds to some redo operation.
Arcs in the tree are ordered so that they always point FROM the current state. When the current state moves from one state to the other, the arc it traversed is reversed. The undo module does this by calling a generic function that client code must supply a method for.