[m-rev.] for review: enforce only abstract instances in interface sections

[Peter Wang]

On Thu, 06 Aug 2026 04:06:06 +0200 "Zoltan Somogyi" <zoltan.somogyi at runbox.com> wrote:
> In the absence of a review by then, I intend to commit this
> in about ten hours. For (possibly post-commit) review by anyone.
> 
> Zoltan.

> Encode an invariant in a type.

That looks fine.

Peter