For more comments, read the patches@python.org archives.
For documentation read the comments in mymalloc.h and objimpl.h.
(This is not exactly what Vladimir posted to the patches list; I've
made a few changes, and Vladimir sent me a fix in private email for a
problem that only occurs in debug mode. I'm also holding back on his
change to main.c, which seems unnecessary to me.)
The Setup.in entry is sort of a lie; it links with -lexpat, but
Expat's Makefile doesn't actually build a libexpat.a. I'll send
Expat's author a patch to do that; if he doesn't accept it, this
rule will have to list Expat's object files (ick!), or have a
comment explaining how to build a .a file.