intros.
induction  n as [| n Hrecn].
simpl in |- *.
apply Refl.
unfold sum at 1 in |- *.
apply SGROUP_comp; auto with algebra.
apply sum_comp.
generalize dependent (Seqtl_cons_inv m f); intro p.
apply p.
Qed.
Hint Resolve sum_cons: algebra.

Lemma sum_concat :
forall (n m : Nat) (G : monoid) (a : seq n G) (b : seq m G),
sum (a ++ b) =' sum a +' sum b in _.