qp.hlp (Topic list)
Important Notice
The pages on this site contain documentation for very old MS-DOS software, purely for historical purposes. If you're looking for up-to-date documentation, particularly for programming, you should not rely on the information found here, as it will be woefully out of date.
Undo Command
 
  Use the Undo command to retract your most recent edit.
  The shortcut key is ALT+BACKSPACE. The Undo command
  works only while the cursor is still on the edited line.
  Once you move the cursor off that line, Undo no longer
  has effect.