public String undo(){
  if (overrideResource != null) {
    return overrideResource.undo();
  }
  if (isUndoable()) {
    UndoableEdit edit=undoQueue.remove(undoQueue.size() - 1);
    redoQueue.add(edit);
    return edit.undoAction();
  }
  return null;
}

public String undo ( ) { if ( overrideResource != null ) { return overrideResource . undo ( ) ; } if ( isUndoable ( ) ) { UndoableEdit edit = undoQueue . remove ( undoQueue . size ( ) - 1 ) ; redoQueue . add ( edit ) ; return edit . undoAction ( ) ; } return null ; }
undo the last operation
