induction ls; intuition; simpl in *.
destruct (eqd a2 a); subst.
intuition.
simpl in *.
intuition.
right.
eapply IHls; eauto.
Qed.
(* ls1 is the smaller set.  This function returns a permutation of ls2 where the first elements of ls2 match the order of the elements in ls1.  This function only works if ls1 is a subset of ls2. *)
Fixpoint matchOrder (A : Set)(eqd : eq_dec A)(ls1 ls2 : list A) :=
match ls1 with
| nil => ls2
| a :: ls1' =>
a :: (matchOrder eqd ls1' (removeFirst eqd ls2 a))
end.

Lemma matchOrder_In : forall (A : Set)(eqd : eq_dec A)(ls1 ls2 : list A),
NoDup ls1 ->
NoDup ls2 ->
(forall a, In a ls1 -> In a ls2) ->
(forall a, In a ls2 <-> In a (matchOrder eqd ls1 ls2)).