Coercion seq_In T : seq T -> set T := fun s x => List.In x s.Coercion list_In T : list T -> set T := fun s x => List.In x s.

Lemma setU_l_subset {U} (s1 s2 s3 : set U) : s1 \subset s3 -> s2 \subset s3 -> (s1 :|: s2) \subset s3.