Can a federal district court exclude a witness in a civil case?

We next consider the district court's exclusion of the government's witness. In United States v. Gatto, 763 F.2d 1040 (9th Cir.1985), this court held that the separation of powers principle limits the supervisory powers of the district courts in government cases. Consequently, a district court can exclude otherwise admissible government evidence only when a violation of "any constitutional provision, federal statute, specific discovery order, or any other recognized right" has been shown. Id. at 1046.