proposition: Lemma setU_subset_l {U} (s1 s2 s3 : set U) : s1 \subset s2 -> s1 \subset (s2 :|: s3).
proof: Proof. firstorder. Qed.
imports: mathcomp.ssreflect.ssreflect Classes.RelationClasses Classes.Morphisms List Tactics mathcomp(ssrfun ssrbool ssrnat seq)