Thank you Richard, Lennart, Gergő

> pattern Positive <- ((>0) -> True)

Heh heh, there's another surprise/undocumented 'feature'.

It's not necessary to give a signature for pattern `Positive`, GHC will infer that from the decl.

I was surprised to see `True`, and even more surprised there wasn't a `Bool` in the signature.  I guess that's so `Positive` can appear as a pattern in a case expr. To dig out the positive value in a lambda expr, it seems I go

> (\p@Positive -> p) 5         -- returns 5

Seems I can't use any trick like that to turn `Positive` into explicitly bidirectional. I also tried

> pattern Positive' <- ((>0) -> ())

But that's rejected  '"* Couldn't match expected type `Bool' with actual type `()'".
Is the hidden `Bool` documented somewhere? (Doesn't seem to be in the User Guide nor the wiki nor the paper, on a quick scan.)