proposition: Lemma setU_set_incl_r : forall (T : Type) (s1 s2 s2' : set T), s1 \subset s2' -> s1 \subset s2 :|: s2'.
proof: Proof. now firstorder. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)