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

Lemma incl_flat_map (A B : Type) (f g : A -> list B) l : (forall a, In a l -> incl (f a) (g a)) -> incl (flat_map f l) (flat_map g l).