For example, save file is not undoable, but is queued in the list to show that it was executed. But there are some actions which are not stored in the undo list, and thus they cannot be undone. Usually undo is available until the user undoes all executed operations. The expectations for undo are easy to understand: to have a predictable functionality, and to include all 'undoable' commands. With the possibility of undo, users can explore and work without fear of making mistakes, because they can easily be undone. In some more advanced programs, such as graphic processing, undo will negate the last command done to the file being edited. It erases the last change done to the document, reverting it to an older state.
Undo is an interaction technique which is implemented in many computer programs.
