What is the evidentiary burden of producing evidence to support a contemnor's claim that it is factually impossible for the contemnor to comply?

4 In cases where the contemnor claims that it is factually impossible for him to comply, he must bear the burden of producing evidence to support his claim. See United States v. Rylander, 460 U.S. 752, 757, 103 S.Ct. 1548, 1552, 75 L.Ed.2d 521 (1983). But in a case such as this where the facts are not disputed and the question is the contemnor's legal authority to comply, there is no need to allocate evidentiary burdens, since the question is one of law.