> 
>> Then should we add a mapping for the buffer-undo-list to t at that
>> point?  Or should we just do nothing?
> 
> Good question.  I think you have a better understanding of how the equiv
> table should be filled than I do at this point, so I'd trust your judgment.

Ok, I looked into it in detail, undo-equiv-table is also used to check if the previous command is really an undo, alongside with checking last-command. So the undo record has to map to something, I decide to map it to ‘empty unless there is already a mapping for the record. Here is the (standalone) final draft. Please have a look.

Yuan