(** Induction on the left transitive step *)

(* Specifications: *)
Lemma clos_refl_trans_ind_left :
forall (x:A) (P:A -> Prop), P x ->
(forall y z:A, clos_refl_trans R x y -> P y -> R y z -> P z) ->
forall z:A, clos_refl_trans R x z -> P z.