Fixpoint sigma (l1 : list E) : (E -> Z) -> Z :=
  fun f : E -> Z =>
  match l1 with
  | nil => 0%Z
  | e :: l2 => (f e + sigma l2 f)%Z
  end.

Lemma sigma_null : forall l : list E, sigma l (fun e : E => 0%Z) = 0%Z.