What is the test for defining reasonable doubt?

We recognized the difficulty of defining reasonable doubt in United States v. Witt, 648 F.2d 608 (9th Cir.1981), where we held that a district court's refusal to define reasonable doubt was not reversible error. We stated that "[a]lthough a proper definition is always appropriate, the decision whether to define reasonable doubt should be left to the court's discretion." Id. at 610-11 (footnote omitted).