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

[Yuan Fu]

> 
> 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

undo-in-region-2.patch
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