What is the test for a search warrant that refers to illegal activity?

United States v. Spilotro, 800 F.2d 959, 963 (9th Cir.1986) (citations omitted). "Reference to a specific illegal activity can, in appropriate cases, provide substantive guidance for the officer's exercise of discretion in executing the warrant." Id. at 964. The warrant in this case was sufficiently specific because it referred to the particular illegal activity which the seized documents must evidence. See United States v. Hernandez-Escarsega, 886 F.2d 1560, 1567-68 (9th Cir.1989), cert. denied, 497 U.S. 1003 (1990); United States v. Fannin, 817 F.2d 1379, 1383 (9th Cir.1987).