Proof with eauto.
intros s s' H.
induction H; intros ? ??...
- eapply eqTrans. eapply eqStep. eapply stepAppL. eassumption.
induction H0...
- induction H...
Qed.

Lemma unique_normal_forms (s t : term) : lam s -> lam t ->  s ≡ t -> s = t.