> I don't know what that means, or why it appears since I've declared unused
> variables with no issues in the past.
It means that you have to either assign it to a variable or to `discard` it
explicitly or to mark the `proc` as `{.discardable.}`. However, your current
implementation doesn't actually modify any of its argument, and thus it becomes
a no-op if you discard the result. The error message was trying to tell you
that you might be losing important information by discarding it.
As for
> I've declared unused variables with no issues in the past.
It's because declaring an unused variable isn't disallowed by the language
specification, while discarding the result (if any) of a procedure call is.
