proposition: Lemma somes_subset {A} (s1 s2 : set (option A)) : s1 \subset s2 -> somes s1 \subset somes s2.
proof: Proof. intros Hs a. apply Hs. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)