> 
> Thanks a lot (especially for improving the docstring ;-).  In my
> experience, this part of the code is quite delicate and I wouldn't be
> surprised to find yet more bugs for other corner cases.  So I think it's
> very important to document those corner cases with tests.  Any chance
> you could add matching tests to test/lisp/simple-tests.el in your patch?
> 

This patch applies on top of the first one. I added more docstrings since you seem to appreciate it. I came up with some simple test cases for ordinary mapping, t and ‘undo-in-region in the table. Do you have other corner cases in mind? I also added some simple test cases for undo in region. I couldn’t wrap my head around the original tests so I started a new one below.

Yuan