proposition: Lemma env_eq_sum : forall El El' Er Er', El [=] El' -> Er [=] Er' -> El [+] Er [=] El' [+] Er'.
proof: Proof. intros. rewrite H; rewrite H0. apply env_eq_refl. Qed.
imports: CoLoR(RelExtras ListExtras TermsLifting LogicUtil OptUtil) Coq(Arith Setoid Morphisms Lia)