mixin under -betterC

[Julian Fondren]

On Thursday, 23 November 2023 at 17:46:55 UTC, DLearner wrote:
> I just find it surprising that your suggestion worked, but the 
> (slightly simpler) earlier version did not.

The `enum` answer? That also works, but you have to make a change 
at the callsite as well, to `mixin(mxnTest!("Var_A", "Var_B"));` 
- passing the strings as template rather than functional 
arguments.