Instance proper_set_incl A :
Morphisms.Proper (set_eq ==> set_eq ==> Basics.impl) (@set_incl A).
Instance eq_setI U : Proper (set_eq ==> set_eq ==> set_eq) (@setI U).

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