UNDO ARRAYED_HISTORY COMMAND_BEHAVIOR COMMAND_HISTORY COMMAND_HISTORY_ENTRY COMMAND_INSTANCE CONTEXT HISTORY_CONTAINER INSTANTIATED_COMMAND MULTI_LEVEL_UNDO MULTIPLE_COMMAND_ENTRY NON_UNDOABLE_COMMAND SINGLE_COMMAND_ENTRY SINGLE_LEVEL_UNDO UNDO_MECHANISM UNDOABLE_COMMAND