I think it would be great to have the following variant of substitute*: (substitute*-once filename (pattern vars) body)
which acts like the usual substitute-*, except it also asserts that the substitution applies to exactly one line in the file, causing a build failure otherwise. In the cases where this is sufficient (I believe most), it would make substitution work quite a bit more reliably, making it both easier to debug substitution when first packaging, and noticing more easily when they need to be adapted on upgrades. (It would be possible to make the signature a bit more flexible and to allow multiple files or patterns as in substitute*, but that would make the meaning of “applies exactly once” a bit unclear, so I’d prefer to not do that. Similarly, I find it cleaner to potentially call substitute*-once several times in a row with different substitutions to make the ordering of effects explicit.) (I’d be happy to supply a patch myself eventually, but the syntax rule business is a bit out of reach at this point.)
