proposition: Lemma env_subset_sum_leq : forall El Er Er', El [<->] Er' -> envSubset Er Er' -> envSubset (El [+] Er) (El [+] Er').
proof: Proof. intros. apply env_subset_sum; trivial. apply env_subset_refl. Qed.
imports: CoLoR(RelExtras ListExtras TermsLifting LogicUtil OptUtil) Coq(Arith Setoid Morphisms Lia)