proposition: Lemma normal_form_dec M : (normal_form M) \/ not (normal_form M).
proof: Proof. case: (step_dec M). - move=> [N] /normal_form_not_step HMN. by right. - move=> /not_step_normal_form ?. by left. Qed.
imports: Undecidability.LambdaCalculus.Lambda PeanoNat Lia List Relations ssreflect