Could you explain what you mean by:

> It is expressible as _not_ a GADT, but it is misleading what the forall is, so I don't write them that way
>
> oh and originally it was any type of content, so it was existensial, so it was a GADT. But restricting it is better

I like the GADT syntax, but I'm not sure how `k` is restricted at all here.