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