(* Being a subtree of *)

Fixpoint InCBV p proof_tree : Prop :=
  p = proof_tree \/
  match proof_tree with
      | cbv_constr lp _ _ => orl (map (InCBV p) lp)
      | cbv_split lp p' _ _ => InCBV p p' \/ orl (map (InCBV p) lp)
      | cbv_function _ _ p' _ _ => InCBV p p'
  end.

(* Specifications: *)
Lemma InCBV_refl p : InCBV p p.
Lemma InCBV_trans p p' p'': InCBV p p' -> InCBV p' p'' -> InCBV p p''.
Lemma cbv_reverse_induction : forall (P : cbv -> Prop) proof_tree, P proof_tree -> (forall lp t v, InCBV (cbv_constr lp t v) proof_tree -> P (cbv_constr lp t v) -> forall p, In p lp -> P p) -> (forall lp p t v, InCBV (cbv_split lp p t v) proof_tree -> P (cbv_split lp p t v) -> forall p', (p' = p \/ In p' lp) -> P p') -> (forall i s p t v, InCBV (cbv_function i s p t v) proof_tree -> P (cbv_function i s p t v) -> P p) -> forall p, InCBV p proof_tree -> P p.