Here's a minimal first cut at describing Py_DEBUG and
friends.
Hopefully the description is detailed enough to be
useful but not so
detailed as to always be out-of-date.  If nothing else,
perhaps it
will spur someone with better knowledge of these
details to correct my
mistakes and flesh out detail.

As is usual for me, I have not run this through LaTeX.
 (I lack it at
work.)  It will need to be checked for correctness.

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