The art of guessing a packaged downgrade version of a package

[swedebugia@riseup.net]

(trying to get gnupg to work knowing that ludo runs some 2.0 version)
~/src/guix$ guix package -i gnupg:2.0.0 -r gnupg
guix package: error: package `gnupg-2.1.10' lacks output `2.0.0'

I would like to improve this command by making it either show me the 
valid outputs (and optionally choosing one of them to install) for this 
specific package OR hint me about running:
guix package --show=gnupg
~$ guix package --show=gnupg
name: gnupg
version: 2.1.10
outputs: out
...

name: gnupg
version: 2.0.29
outputs: out
...

name: gnupg
version: 1.4.20
outputs: out

to get the information I need.

I found the relevant code in:
../gnu/packages.scm:381:        (leave (_ "package `~a' lacks output 
`~a'~%")

To be able myself to improve this snippet I would have to learn a lot 
more Guile first. :p

Anybody care to help?

swedebugia