Introspecting source registry

[Mark Evenson]

With asdf-3.1.4, how do I introspect the current state of source
registry configuration?  I need to debug a remote user's ASDF
configuration on a machine which I don't have access to, so I would like
to have ASDF output the current source registry configuration search
state.  Problematically, the behavior documented in section 8.1 doesn't
seem to be working as described on the user's machine.  And this
behavior seemingly changes between "patch level" ASDF releases
especially on marginal platforms like Windows, digging up the "correct"
version of the manual isn't so easy either.

[1]:
https://common-lisp.net/project/asdf/asdf.html#Controlling-where-ASDF-searches-for-systems

The contents of ASDF/SOURCE-REGISTRY:*SOURCE-REGISTRY* provides a
hashtable of systems that ASDF has located, but doesn't help me figure
out where ASDF *might* have searched for things.  For output
translations the return value of (ASDF:INITIALIZE-OUTPUT-TRANSLATIONS)
gives me enough information for output translations, so it would be nice
to have something equivalent for the source registry.  Since I suppose
that all of the behavior specified in section 8.1 is not easily
transcribed as a s-expr, my request may not be trivial to implement, but
in lieu of some similar mechanism, I am really having problems with
debugging ASDF behavior in a particular instance.

Additionally, what is the behavior of ASDF when the $XDG_* environment
variables are not set, as is seemingly the case on a stock Windows
environment?  Are these configuration steps just skipped?

-- 
"A screaming comes across the sky.  It has happened before, but there
is nothing to compare to it now."