The <Undo> key (or shift-F2 on some systems) can also be used to restore a buffer which was cleared using the Shift-F9 key. Whenever a buffer is cleared, a copy of the file cleared is placed in the undo buffer. Since this buffer takes up space in some of the computer's memory, if you are short on memory, you may want to hit shift-F9 twice when you are clearing a buffer. The first time will move the file into the undo buffer, and the second time will copy the contents of the (now empty) buffer into the undo buffer, thus effectively clearing it and freeing up the memory allocated to it.
Note: For all the above operations, there is only one undo buffer for all files. Thus if you make a deletion in one file and then move to another to do some editing, the information to undo the delete in the first file will be lost. This is a compromise to the fact that there are still so many computers in which memory is a hard item to get. In the future this restriction may be lifted and a more versatile undo will be implemented.