Wed Jan  3 13:51:25 PST 2018
There are 4 versions of xz_mod here. Or 5 in a way.

xz_mod.py contains all four, and prefers them in an order it thinks will be
best, but isn't always.  It is the best tested version of xz
compression/uncompression.

xz_base_mod.py has an abstract class used by the 4, but is otherwise unrelated
to xz_mod.py.

xz_ctypes_mod.py, xz_lzma_mod.py, xz_os_mod.py and xz_subprocess_mod.py are the
four; each uses xz_base_mod.py.  They each contain 1 way of doing xz
compression and decompression.  They are derived from xz_mod.py, but are unused
at this time.

I've been considering copying or symlinking 1 of these 4 to xz_mod.py, to keep
things simple, and give the sysadmin greater control over what's happening than
what xz_mod.py currently allows.

However, the control doesn't appear to be entirely necessary.  xz_lzma_mod.py
is quite good in most cases.  True, it's really slow on some of the older
pypy3's, but recently it's been doing fine, even on pypy3.

To be absolutely clear: 
xz_ctypes_mod.py, xz_lzma_mod.py, xz_os_mod.py and xz_subprocess_mod.py are
each 1 adapter for the same things that xz_mod.py has 4 adapters for.