Description: The Rhino Undo command will call this virtual Undo function when the event needs to be undone. You should override this function to undo changes in your plug-in data,
NEVER change any setting in the Rhino document or application. Rhino handles ALL changes to the application and document and you will break the Undo/Redo commands if you make any changes to the application or document. Parameters: doc - [in] Document in which this undo is taking place. cmd - [in] Command being undone. Can be NULL. action_description - [in] Description of what is being undone. Can be NULL. bCreatedByRedo - [in] True if a "Redo" was responsible for creating this record. undo_event_sn - [in] Runtime serial number Rhino assigned to the event.