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