proposition: Lemma imset_f (D : {pred aT}) x : x \in D -> f x \in f @: D.
proof: Proof. by move=> Dx; apply/imsetP; exists x. Qed.
imports: HB(structures) mathcomp(ssreflect ssrbool ssrfun eqtype ssrnat div seq) mathcomp(choice fintype finfun bigop)