Re: Distinguish between regional undo and undo to the beginning in undo-equiv-table

[Yuan Fu]

BTW, I can’t quite get what the comment at simple.el:2920 means:

(puthash list
               ;; Prevent identity mapping.  This can happen if
               ;; consecutive nils are erroneously in undo list.
               (if (or undo-in-region (eq list pending-undo-list))
                   t
                 pending-undo-list)
               undo-equiv-table)

Is it that if there is (nil nil nil) on the top of buffer-undo-list, the middle 
one will be considered an undo record and will be passed to primitive-undo? In 
that case nothing is done and nothing is added to buffer-undo-list? Then should 
we add a mapping for the buffer-undo-list to t at that point? Or should we just 
do nothing?

Yuan