Remark le_sigma_sigma :
forall (l : list E) (f g : E -> Z),
(forall x : E, (f x <= g x)%Z) -> (sigma l f <= sigma l g)%Z.

(* Specifications: *)
Lemma ge_sigma_sigma :
forall (l : list E) (f g : E -> Z),
(forall x : E, (f x >= g x)%Z) -> (sigma l f >= sigma l g)%Z.