proposition: Lemma env_subset_sum_l : forall E El Er, El [<->] Er -> envSubset E El -> envSubset E (El [+] Er).
proof: Proof. intros; intros x A D. apply env_sum_ly. exact (H x). apply H0; trivial. Qed.
imports: CoLoR(RelExtras ListExtras TermsLifting LogicUtil OptUtil) Coq(Arith Setoid Morphisms Lia)