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