proposition: Lemma ok_middle_change : forall E F x v1 v2, ok (E & x ~ v1 & F) -> ok (E & x ~ v2 & F).
proof: Proof. Proof using. induction F using env_ind; introv; rew_env_concat; introv Ok. destruct* (ok_push_inv Ok). destruct* (ok_push_inv Ok). Qed.
imports: TLC(LibTactics LibOption LibList LibProd LibLogic LibReflect)