Undo is an interaction technique which is implemented in many computer programs. It erases the last change done to the document, reverting it to an older state. In some more advanced programs, such as graphic processing, undo will negate the last command done to the file being edited. With the possibility of undo, users can explore and work without fear of making mistakes, because they can easily be undone. The expectations for undo are easy to understand: to have a predictable functionality, and to include all "undoable" commands. Usually undo is available until the user undoes all executed operations. But there are some actions which are not stored in the undo list, and thus they cannot be undone. For example, save file is not undoable, but is queued in the list to show that it was executed. Another action which is usually not stored, and thus not undoable, is scrolling or selection. The opposite of undo is redo. The redo command reverses the undo or advances the buffer to a more recent state. The common components of undo functionality are the commands which were executed of the user, the history buffer(s) which stores the completed actions, the undo/redo manager for controlling the history buffer, and the user interface for interacting with the user. In most Microsoft Windows applications, the keyboard shortcut for the undo command is Ctrl+Z or Alt+Backspace, and the shortcut for redo is Ctrl+Y or Ctrl+Shift+Z. In most Apple Macintosh applications, the shortcut for the undo command is Command-Z, and the shortcut for redo is Command-Shift-Z. On all platforms, the undo/redo functions can also be accessed via the Edit menu. History The ability to undo an operation on a computer was independently invented multiple times, in response to how people used computers. The File Retrieval and Editing System, developed starting in 1968 at Brown University, is reported to be the first computer-based system to have had an "undo" feature. Warren Teitelman developed a Programmer's Assistant as part of BBN-LISP with an Undo function, by 1971. The Xerox PARC Bravo text editor had an Undo command in 1974. A 1976 research report by Lance A. Miller and John C. Thomas of IBM, Behavioral Issues in the Use of Interactive Systems, noted that "it would be quite useful to permit users to 'take back' at least the immediately preceding command (by issuing some special 'undo' command)." The programmers at the