Re: New undo element (fun . args)

[Richard Stallman]

      /* Make sure this produces at least one undo entry,
         so the test in `undo' for continuing an undo series
         will work right.  */
      if (EQ (oldlist, current_buffer->undo_list))
        current_buffer->undo_list
          = Fcons (list3 (Qapply, Qcdr, nil), current_buffer->undo_list);


    I don't understand why this is necessary.  The primitive-undo
    continues to the next undo boundary, so why is it important what is
    recorded "between undo boundaries" ?

I think it is necessary for this code to work:

                     ;; If something (a timer or filter?) changed the buffer
                     ;; since the previous command, don't continue the undo seq.
                     (let ((list buffer-undo-list))
                       (while (eq (car list) nil)
                         (setq list (cdr list)))
                       ;; If the last undo record made was made by undo
                       ;; it shows nothing else happened in between.
                       (gethash list undo-equiv-table))))

If undoing does not generate any undo entries, there will be nothing
for this hash mark to ride on.  However, I may have misunderstood
something.

      It works
    to some extent, but as soon as undo hits one of the entries I added to
    the undo list, it gets stuck -- and the pending undo list seems to
    be stuck at that specific (apply cdr ...) entry.

Could you debug precisely what is happening when it is "stuck" as you
say?  In what way does the undo process fail to work?  Where is the bug?