(***)
induction k. intuition. intros. unfold nsum;fold nsum. apply
Trans with(C+nsum k(Seqtl v)). apply pluscomp2. unfold head.
apply H. rewrite plus_comm. apply Trans with(k*C+C). apply
pluscomp2. apply IHk. induction i. simpl. apply H. simpl.
rewrite plus_comm. auto. Qed.

Lemma G2Stab:forall(G:group)(S:Setoid)(f:Map(cart G S)S)
(LA1:forall(s:S),f(couple(zero G)s)='s)
(LA2:forall(g1 g2:G)(s:S),
f(couple(g1+' g2)s)='f(couple g1(f(couple g2 s))))
(p:S)(g:G),Stab2_s G S f LA1 LA2 p.