proposition: Lemma env_eq_empty_none_empty : EmptyEnv [=] None :: EmptyEnv.
proof: Proof. intros; split. apply env_subset_empty. intros w A wA. destruct w; try_solve. destruct w; try_solve. Qed.
imports: CoLoR(RelExtras ListExtras TermsLifting LogicUtil OptUtil) Coq(Arith Setoid Morphisms Lia)