Inductive value : tm -> Prop :=
  | v_const : forall n, value (C n).Inductive value : tm -> Prop :=
  | v_const : forall n, value (C n).Instance dec_value t : Dec (value t).Inductive step : tm -> tm -> Prop :=
  | ST_PlusConstConst : forall n1 n2,
          P (C n1) (C n2)
      ===> C (n1 + n2)
  | ST_Plus1 : forall t1 t1' t2,
        t1 ===> t1' ->
        P t1 t2 ===> P t1' t2
  | ST_Plus2 : forall v1 t2 t2',
        value v1 ->                     (* <----- n.b. *)
        t2 ===> t2' ->
        P v1 t2 ===> P v1 t2'

  where " t '===>' t' " := (step t t').Inductive step : tm -> tm -> Prop :=
  | ST_PlusConstConst : forall n1 n2,
          P (C n1) (C n2)
      ===> C (n1 + n2)
  | ST_Plus1 : forall t1 t1' t2,
        t1 ===> t1' ->
        P t1 t2 ===> P t1' t2
  | ST_Plus2 : forall v1 t2 t2',
        value v1 ->                     (* <----- n.b. *)
        t2 ===> t2' ->
        P v1 t2 ===> P v1 t2'

  where " t '===>' t' " := (step t t').Inductive step : tm -> tm -> Prop :=
  | ST_PlusConstConst : forall n1 n2,
          P (C n1) (C n2)
      ===> C (n1 + n2)
  | ST_Plus1 : forall t1 t1' t2,
        t1 ===> t1' ->
        P t1 t2 ===> P t1' t2
  | ST_Plus2 : forall v1 t2 t2',
        value v1 ->                     (* <----- n.b. *)
        t2 ===> t2' ->
        P v1 t2 ===> P v1 t2'

  where " t '===>' t' " := (step t t').Inductive step : tm -> tm -> Prop :=
  | ST_PlusConstConst : forall n1 n2,
          P (C n1) (C n2)
      ===> C (n1 + n2)
  | ST_Plus1 : forall t1 t1' t2,
        t1 ===> t1' ->
        P t1 t2 ===> P t1' t2
  | ST_Plus2 : forall v1 t2 t2',
        value v1 ->                     (* <----- n.b. *)
        t2 ===> t2' ->
        P v1 t2 ===> P v1 t2'

  where " t '===>' t' " := (step t t').

Theorem step_deterministic : deterministic step.