What is the test for determining whether Fabozzi was responsible for failing to produce a tax return for purposes of being held liable for unpaid withholding taxes?

We have jurisdiction pursuant to 28 U.S.C. Sec. 1291. The district court's finding that Fabozzi was "responsible" for purposes of being held liable for unpaid withholding taxes is on a question of fact and cannot be set aside unless clearly erroneous. Maggy v. United States, 560 F.2d 1372, 1375 (9th Cir.1977), cert. denied, 439 U.S. 821, 99 S.Ct. 86, 58 L.Ed.2d 112 (1978). The district court's finding of willfulness is a finding of fact subject to reversal only if clearly erroneous. Dudley v. United States, 428 F.2d 1196, 1200 (9th Cir.1970). Under the clearly erroneous standard, the appellate court must accept the lower court's findings of fact unless upon review it is left with the definite and firm conviction that a mistake has been committed. Maggy, 560 F.2d at 1375.