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.

Theorem NoDup_count_occ' l: NoDup l <-> (forall x:A, In x l -> count_occ decA l x = 1).