(** *** Incremental proofs using our [pcofix]  As before, we can easily turn the above [cofix]-proofs into [pcofix]-proofs. *)
Hint Unfold teq'.

(** The following proof would fail guardedness checking at [Qed]-time, because in the proof of the lemma [teq_two_one_bad] the assumption is used "too early". *)
Theorem teq_eins_bad : teq one eins.