proposition: Lemma Sum_strext' : forall f g m n, Sum m n f [#] Sum m n g -> {i : nat | f i [#] g i}.
proof: Proof. intros f g m n H. unfold Sum, Sum1 in H. elim (cg_minus_strext _ _ _ _ _ H); intro H1; elim (Sum0_strext _ _ _ H1); intros i Hi Hi'; exists i; assumption. Qed.
imports: CoRN.algebra.CAbGroups Coq.Arith.Peano_dec Coq(Lia)