In Delaware v. Fensterer, 474 U.S. 15, 106 S. Ct. 292, 88 L. Ed. 2d 15 (1985)(per curiam), the Court determined that there was no Confrontation Clause violation when an expert witness testified as to what opinion he had formed, but could not recollect the basis on which he had formed it. The Court said: "The Confrontation Clause includes no guarantee that every witness called by the prosecution will refrain from giving testimony that is marred by forgetfulness, confusion, or evasion. To the contrary, the Confrontation Clause is generally satisfied when the defense is given a full and fair opportunity to probe and expose these infirmities through cross-examination, thereby calling to the attention of the factfinder the reasons for giving scant weight to the witness' testimony." Id., at 21-22, 106 S. Ct. , at 295. The opinion noted that a defendant seeking to discredit a forgetful expert witness is not without ammunition, since the jury may be persuaded that "his opinion is as unreliable as his memory." Id., at 19, 106 S. Ct. , at 294.