What is the test for cross-examining a defense witness regarding their failure to come forward to the police prior to trial?

Unlike the cases relied on by defendant, People v. Ratliff (1987) 189 Cal.App.3d 696, 234 Cal.Rptr. 502, concerns a prosecutor's cross-examination of defense witnesses regarding their failure to volunteer exculpatory information to the police prior to trial. The court held the inquiry to be proper, and we believe rightly so, "since it was designed to test their credibility, knowledge and recollection. [Citations.] There is nothing inherently improper about cross-examining a defense witness as to his failure to come forward at an earlier date. In fact, the information discovered during this type of questioning may well aid the trier of fact in its effort to determine whether the testimony is an accurate reflection of the truth or a recent fabrication. [p] Although a citizen ordinarily has no legal obligation to offer exculpatory information to law enforcement officials, there are many situations where the natural response of a person would be to come forward in order to avoid a mistaken prosecution of a relative or friend. In that situation[222 Cal.App.3d 737] a witness's silence may be akin to a 'prior inconsistent statement,' and therefore, has probative value. [Citation.]" (Id. at pp. 700-701, 234 Cal.Rptr. 502.)