reset-undo! -- Reset the undo system.
increment-in-range -- Return v+1 but no more than high.
decrement-in-range -- Return v-1 but no less than low.
decrement-undo-index -- Decrement the undo-index.
increment-undo-index -- Increment the undo-index.
push-undo-global -- Push the global state onto the undo list.
insert-undo-global -- Insert the global state into the undo list at undo-index.
push-undo-state -- Push STATE onto the undo list.
insert-undo-state -- Insert STATE into the undo list at undo-index.
undo-state-at-index -- Return the state from undo-list at INDEX, or #f if bad index.
undo -- Undo the last operation that was undoable.
redo -- Redo the last undone operation.
undo-use -- Apply the state from the undo-list contained at INDEX.