proposition: Lemma set_eq_isSome_sound {A} (s : set (option A)) (s' : set A) : isSome :&: s <--> Some @: s' -> s \subset (Some @: s') :|: [set None].
proof: Proof. intros H [x| ] Hin. - left. eapply H. eexists; eauto. - right; reflexivity. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)