> For simplicity, change this pattern to ("1" id body).  That will allow
> the inner ‘cond’ to be simplified.

I like informative error messages, may I keep it please?  The attached
diff should address all the things you mentioned except this one.
Please review.

I’m planning to send a proper patch as soon as I test (guix base64) and
change a couple of things in (test-substitute-binary).