In Cool v. United States, 409 U.S. 100 (1972), an accomplice testified that he acted alone, and that the defendant neither knew about nor participated in the criminal conduct. The trial court instructed the jury that it could accept this accomplice testimony only if it found it true beyond a reasonable doubt. (Cool, supra, 100 U.S. at pp. 101-102.) The United States Supreme Court overturned the conviction because the instruction improperly affixed the burden of proof on the accomplice's credibility "by creating an artificial barrier to the consideration of relevant defense testimony." (Id. at p. 104) The trial court instructed the jury that accomplice testimony exculpating the defendant was "open to suspicion" and should only be treated like other evidence if it was proven beyond a reasonable doubt. (Id. at p. 102.) The Supreme Court held that this instruction effectively told the jury to disregard the accomplice testimony unless it was considered true beyond a reasonable doubt. As such, the instruction interfered with the defendant's right to defend with evidence that merely raised a reasonable doubt and, in doing so, lowered the prosecution's burden of proof. (Id. at p. 104.) At a trial for possession of counterfeit bills, an accomplice testified for the defense that he was guilty of the offense but the defendant was not involved. (Id. at pp. 100-101.) The trial court instructed the jury that "an accomplice's testimony is 'open to suspicion,'" but also that if the jury was convinced "beyond a reasonable doubt" that the accomplice's testimony was true, it should be given the same effect as any other witness's testimony. (Id. at p. 102.) In explaining the propriety of accomplice instructions when the accomplice testifies for the prosecution, because the instructions represent "no more than a commonsense recognition that an accomplice may have a special interest in testifying, thus casting doubt upon his veracity," Cool stated that "no constitutional problem is posed when the judge instructs a jury to receive the prosecution's accomplice testimony 'with care and caution.'" (Cool, supra, 409 U.S. at p. 103.) The court went on to say, "But there is an essential difference between instructing a jury on the care with which it should scrutinize certain evidence in determining how much weight to accord it and instructing a jury, as the judge did here, that as a predicate to the consideration of certain evidence, it must find it true beyond a reasonable doubt." (Id. at p. 104.) The instruction given by the trial court in Cool impermissibly obstructed the defendant's exercise of her right to present exculpatory evidence from an accomplice "by totally excluding relevant evidence unless the jury makes a preliminary determination that it is extremely reliable." (Ibid.) Additionally, the court explained, by telling the jury it could not consider the accomplice's testimony unless it found that testimony believable beyond a reasonable doubt, the instruction reduced the prosecution's burden of proof "by creating an artificial barrier to the consideration of relevant defense testimony putatively credible by a preponderance of the evidence." (Ibid.) The Court explained that the defendant's constitutional right to present exculpatory evidence of an accomplice was violated not because an accomplice instruction was given, but because the instruction impermissibly excluded relevant evidence unless the jury makes a preliminary determination that it is extremely reliable, and had the effect of reducing the prosecution's burden of proof. The Court implicitly held that an accomplice testimony instruction directed at the testimony of a defense witness does not constitute constitutional error. See id. at 103. The Court observed that, in general, accomplice instructions "represent no more than a commonsense recognition that an accomplice may have a special interest in testifying, thus casting doubt upon his veracity." Id.