Non-polymorphic functions of the type `a->a` trivially satisfy it, of course. Also, I think polymorphic functions are the most interesting case where you can't

Non-polymorphic functions of the type `a->a` trivially satisfy it, of course. Also, I think polymorphic functions are the most interesting case where you can't use twice as desired in an ML-like language, so it's too bad that despite the fancier type they don't work too well in MLsub. Here's another example that fails when I'd hope for it to succeed: let one = twice (fun o -> o.x) { x = { x = 1 } }