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