Hi Fabio, everyone.

I can reproduce the defect and can confirm that the patch solves the 
problem.

I don't have commit access, but, for what it's worth, I see that in a 
|./pre-inst-env|

☑ The patch applies correctly
☑ |guix build --check --no-grafts guile-proba |succeeds
☑ guix lint guile-proba does not report issues (although it indicates 
the package can be upgraded to 0.3.1)

Cheers,