induction n.
intros.
simpl in |- *.
apply Refl.
intros.
unfold sum in |- *.
apply SGROUP_comp; auto with algebra.
apply IHn.
change (Seqtl f =' Seqtl f' in _) in |- *.
apply Seqtl_comp; auto with algebra.
Qed.
Hint Resolve sum_comp: algebra.

(* Specifications: *)
Lemma sum_cons :
forall (M : monoid) (m : M) (n : Nat) (f : seq n M),
sum (m;; f) =' m +' sum f in _.