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

Lemma suml_flat_map_map (A B : Type) (f : A -> list B) (g : B -> nat) l : suml (flat_map (fun x => map g (f x)) l) = suml (map (fun x => suml (map g (f x))) l).