What is the test for determining reasonable doubt in a sexual assault case?

Subsequently, in United States v. Wosepka, 757 F.2d 1006 (9th Cir.), modified, 787 F.2d 1294 (1985), we explained our holding in Witt. We held that while the district court has the discretion to refuse to give an instruction defining reasonable doubt if "the issues to be resolved by the jury ... [are] fairly staightforward and the evidence [is] not complex," id. at 1009, the court may not refuse to do so if the case is one of any complexity.