(** Earlier, the Peano-like recursor was built and proved in a way due to
    Conor McBride, see "The view from the left" *)

Inductive PeanoView : positive -> Type :=
| PeanoOne : PeanoView 1
| PeanoSucc : forall p, PeanoView p -> PeanoView (succ p).(** Earlier, the Peano-like recursor was built and proved in a way due to
    Conor McBride, see "The view from the left" *)

Inductive PeanoView : positive -> Type :=
| PeanoOne : PeanoView 1
| PeanoSucc : forall p, PeanoView p -> PeanoView (succ p).(** Earlier, the Peano-like recursor was built and proved in a way due to
    Conor McBride, see "The view from the left" *)

Inductive PeanoView : positive -> Type :=
| PeanoOne : PeanoView 1
| PeanoSucc : forall p, PeanoView p -> PeanoView (succ p).Fixpoint peanoView_xO p (q:PeanoView p) : PeanoView (p~0) :=
  match q in PeanoView x return PeanoView (x~0) with
    | PeanoOne => PeanoSucc _ PeanoOne
    | PeanoSucc _ q => PeanoSucc _ (PeanoSucc _ (peanoView_xO _ q))
  end.Fixpoint peanoView_xI p (q:PeanoView p) : PeanoView (p~1) :=
  match q in PeanoView x return PeanoView (x~1) with
    | PeanoOne => PeanoSucc _ (PeanoSucc _ PeanoOne)
    | PeanoSucc _ q => PeanoSucc _ (PeanoSucc _ (peanoView_xI _ q))
  end.Fixpoint peanoView p : PeanoView p :=
  match p return PeanoView p with
    | 1 => PeanoOne
    | p~0 => peanoView_xO p (peanoView p)
    | p~1 => peanoView_xI p (peanoView p)
  end.Definition PeanoView_iter (P:positive->Type)
  (a:P 1) (f:forall p, P p -> P (succ p)) :=
  (fix iter p (q:PeanoView p) : P p :=
    match q in PeanoView p return P p with
      | PeanoOne => a
      | PeanoSucc _ q => f _ (iter _ q)
    end).

Lemma mul_cancel_l p q r : r * p = r * q <-> p = q.