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_NoDup (l l' : list A) : NoDup l -> length l' <= length l -> incl l l' -> NoDup l'.