| Classes that implement this interface provide undo/redo
capability linked to changes reported by model change
providers. Model change events carry sufficient data
to be used in an undo/redo stack and reverted to or
reapplied after the change.
Model undo manager adds itself as a change listener
after being connected to the provider. It is expected
to stop listening to change events after being disconnected.
Changes reported while being connected are kept in the
operation stack whose size can be controlled.
The part that uses the undo manager is responsible
for supplying Undo and Redo action objects for
the purpose of controlling their availability.
Undo manager should keep track of its current
operation stack pointer and adjust Undo/Redo action
availability by calling 'setEnabled' on the
provided action objects. Implementation of this
interface may also opt to modify Undo/Redo action
labels in order to better indicate the effect
of the operations if selected (for example,
'Undo Delete' instead of 'Undo').
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