If we squint a bit, we realize it’s the same thing but in a different
order, which is good news: it’s functionally equivalent.

The downside is obvious: it’s stupidly non-deterministic, and we can end
up building the same grafts multiple times.

The order differs in two places: in the definition of ‘%build-inputs’,
and in the definition of the ‘mapping’ variable.  This can be solved by
sorting things in the right place, but that needs some thought.

To be continued…

Ludo’.