(** Effective computation of a list without duplicates *)
Fixpoint nodup (l : list A) : list A :=
match l with
| [] => []
| x::xs => if in_dec decA x xs then nodup xs else x::(nodup xs)
end.

Lemma nodup_incl l1 l2 : incl l1 (nodup l2) <-> incl l1 l2.