Hi Niels!
Several months ago you asked on IRC about extending the substvars file
format to be able to mark substvars as optional (so that they would not
warn in case they end up not being used, like how several of the built-in
ones are handled). At the time I mentioned that I'd need to ponder about
it but had no energy (or motivation) do work on stuff.
I've not forgotten about it, and it has been on the back of my head,
and I've been converging on using «?=» as operator. Then rechecked the
IRC logs, and noticed that was exactly one of the formats you proposed
at the time (so either it stuck in the background and subconsciously
surfaced, or I came to the same conclusion :). In any case, I've prepared
the attached patch, which I'd be happy to include for 1.21.8, in case
something like this works for you?
Thanks,
Guillem