apply le_lt_trans with (a * d).
apply le_mult_cst.
apply lt_le_weak; assumption.
elim mult_comm; elim mult_comm with d b.
apply lt_mult_cst.
assumption.
apply lt_O with c; auto with arith.
Qed.
Hint Resolve lt_mult_csts.

(* Specifications: *)
Lemma pred_mult : forall n m : nat, 0 < n -> n * m = pred n * m + m.
intros.