intuition.
eapply filter_NoDup.
trivial.
Qed.

Lemma intersect_comm_Permutation :
forall (A : Set)(eqd : eq_dec A)(ls1 ls2 : list A),
NoDup ls1 ->
NoDup ls2 ->
Permutation
(intersect eqd ls1 ls2)
(intersect eqd ls2 ls1).