bug#16581: suggested code simplification in dfa.c

[Paul Eggert]

Aharon Robbins wrote:
> that exclusive-ORing looks a little scary to me.

It is derived from a coding hack that I picked up from Dijkstra back in 
the 1970s.  He gave the most boring computer-science lecture I have ever 
attended -- so boring that Kit Fine walked out a few minutes into it -- 
but I stubbornly stayed through to the end and I've never forgotten the 
hack.

The hack works everywhere, including platforms that use EBCDIC, 
shift-JIS, DBCS, etc., because it doesn't rely on the encoding scheme at 
all.  Attached is a revised patch that adds some commentary and breaks 
the hack into some functions that I hope help explain things.  Sorry, I 
don't know how to break this into smaller patches that would be easier 
to understand.

0001-Simplify-handling-of-letter-case.patch
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