> <\body> It is possible to gradually undo the changes you made in a document from the moment that you launched . This can be done via or using the keystroke . Undone changes can be \Predone\Q using or . maintains the entire \Phistory tree\Q of all your edits. Sometimes, after multiple undo and redo actions, this means that there can be several ways to redo some modification; in that case, becomes a menu in which you can select the appropriate branch. We notice that only changes in the document itself are taken into account by the undo system. In particular, modifications of most of the global document properties can not be undone. This includes modifications of the document style, the page size, the main font, etc. The same remark applies to any modifications outside that were triggered by your actions. For instance, in a computer algebra session, you can undo your edits inside , but not the computations in the external computer algebra system. >