Proof with auto.
induction l...
simpl.
intros.
inversion_clear H...
destruct (d x a); simpl...
subst.
rewrite eq_count_0...
Qed.

(* Specifications: *)
Lemma NoDup_incl_Permutation A (a b: list A):
length a = length b -> NoDup a -> incl a b -> Permutation a b.