Definition fincl A B (AsubB : A `<=` B) (a : A) : B :=
[` (fsubsetP AsubB) _ (valP a)].
Definition fsub B A : {set B} := [set x : B | val x \in A].

Lemma in_fset2 x a b : (x \in [fset a; b]) = (x == a) || (x == b).