Fixpoint suml (l : list nat) : nat :=
  match l with
  | nil => 0
  | n :: l' => n + suml l'
  end.

Lemma suml_map_le : forall (A : Type)(f g : A -> nat)(l : list A), (forall x, In x l -> f x <= g x) -> suml (map f l) <= suml (map g l).