public void generate_snapshot(){
  disable_redo();
  Collection<UndoObjectNode> curr_deleted_objects_list=new LinkedList<UndoObjectNode>();
  deleted_objects_stack.add(curr_deleted_objects_list);
  ++stack_level;
}

public void generate_snapshot ( ) { disable_redo ( ) ; Collection < UndoObjectNode > curr_deleted_objects_list = new LinkedList < UndoObjectNode > ( ) ; deleted_objects_stack . add ( curr_deleted_objects_list ) ; ++ stack_level ; }
makes the current state of the list restorable by undo .
