What is the test for impeaching a witness who has previously admitted that he does not remember the facts of the trial?

People v. Porterfield, 186 Cal.App.2d 149, 159, 8 Cal.Rptr. 897, 903: "The * * * essence of impeachment [of a witness] is that thee is a disagreement between what was formerly said and what was said at the trial."

Anthony v. Hobbie, 85 Cal.App.2d 798, 804, 193 P.2d 748: "Where a witness merely states [on the stand] that he does not remember * * * the introduction of such prior statements would * * * only service to impeach the statement that the witness does not now remember."