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[Arnaud Spiwack <arnaud.spiwack AT gmail.com>]

>> Definition extension2 f x y := exists a, exists b, x a -> x b -> f a b
>
>
> Unfortunately no.
> In Why3, we display a warning whenever the user writes this kind of predicate ; it has been helpful to debug some broken specifications early. It might be worth having the same warning message in Coq.

I'm confused. Apart from the fact that I forgot an obvious lambda (f is supposed to be of type A->B->(X->Prop)), this definition is a correct binary extension for the powerset monad. Could you elaborate on the issue you have with it?

>
> Best regards,
>
> Guillaume
>