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_assoc {U} (s1 s2 s3 : set U) : (s1 :|: (s2 :|: s3)) <--> ((s1 :|: s2) :|: s3).