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).