proposition: Lemma rewrite_env_demo : forall (x y:atom) (T:typ) P, (forall E, P ((x,T):: E) -> True) -> P (x ~ T) -> True.
proof: Proof. intros x y T P H. rewrite_env ((x,T) :: nil). apply H. Qed.
imports: Metalib.Metatheory String