Re: pending-undo-list is not buffer local

[Richard Stallman]

    If those calls are done in different buffers it looks like trouble to 
    me. Should not pending-undo-list be buffer local?

    I think for the reason above that it should be buffer local and I also 
    think that it should be permanent buffer local then.

Maybe so, but in normal Emacs it won't be used again if you
switch buffers.  So we can leave this alone for now.
After the release, let's do what you suggest.