induction ls; destruct ls'; intuition; simpl in *.
intuition.
destruct H; simpl in *; subst; intuition.
destruct H; simpl in *; subst; intuition.
rewrite app_length.
simpl.
omega.
rewrite app_length.
specialize (IHls (a0 :: ls') H).
simpl in *.
intuition.
Qed.
Fixpoint getUnique(A : Set)(ls : list A)(pf : eq_dec A) : list A :=
match ls with
| nil => nil
| a :: ls' =>
let ls'' := (getUnique ls' pf) in
if (in_dec pf a ls'')
then ls''
else a :: ls''
end.

Theorem in_getUnique_if : forall (A : Set)(ls : list A)(eqd : eq_dec A) a,
In a (getUnique ls eqd) ->
In a ls.