ong> is a unit of work that can be executed, undone, and redone.
Operations can be added to an <strong>operation history</strong> so that they can later be
undone and redone according to the undo model for an application.  Operations may be
assigned one or more <strong>undo contexts</strong> which can be used to filter the available
operations for undo or redo in the operation history.  Clients may choose to provide
undo and redo function for all operations in a history, or only for a particular
undo contex