In fixing issue 8692, a change was made to BaseDocument and BaseDocumentEvent
that asserts that the last edit made will be the first edit undone. While quite
sensible, this fails to be useful and does not have anything to do with the bug
fix itself. In fact, it blocks a perfectly reasonable use of the document and
hinders managing its undoable edits.

For the XML Schema editor in the Enterprise Pack, we have a model that is backed
by a text file, which is stored in memory in a BaseDocument instance. The
document is editable by the user via a CloneableEditor. The model is editable
directly via a graphical interface, and changes made to the model are flushed to
the document. Because the model employs a fast tree-based undo/redo support, we
want to manage the model's undoable edits as well as the document's undoable
edits (otherwise, syncing the model with the text is very expensive). This is
described here:

   http://xml.netbeans.org/specs/undo/

As such, we have an undo queue with both document edits and model edits. When
the model edits are undone, they operate on the model and subsequently flush the
change back to the document. The document edits related to this model change are
discarded, as they are not relevant. Anything the model does to the document is
assumed to be perfect and does not need to be 'undone', unlike edits made by the
user.

Because the model flushed its change to the document when its edit was undone,
the BaseDocument lastModifyUndoEdit reference was changed. Subsequently, undoing
any document edit made by the user prior to the model change will result in a
CannotUndoException. As you can imagine, this is a disaster for us. After
spending three weeks developing this undo/redo solution, following Yarda's
explicit directions, we have been crippled by an assertion that, at best,
catches a problem that may never occur.

Another reason why the assertion is of little or no value is that it does not
guard against concurrent changes *after* the last edit is undone. So it only
catches the mistake the first time, but after that all sorts of awful things
could be done. It is of no use and it prevents our perfectly reasonable attempts
at using BaseDocument from succeeding. After all, we are the controller of the
document, we should be allowed to manage the undoable edits and document
contents as we see fit.