Remark return_measure_rec_incr:
  forall f n1 n2 pc,
  (n1 <= n2)%nat ->
  (return_measure_rec n1 f pc <= return_measure_rec n2 f pc)%nat.

Lemma is_return_charact: forall f n pc rret, is_return n f pc rret = true -> (n <= niter)%nat -> is_return_spec f pc rret.