What is the test for impeach testimony in a criminal case?

Whether a question is asked of a codefendant to lay a foundation for testimony, or a question is asked of a codefendant to impeach testimony, the concern is the same: "The government may not introduce evidence of a co-defendant's conviction as substantive evidence of the guilt of the defendant who is on trial." United States v. Peterman, 841 F.2d 1474, 1479 (10th Cir.1988) (finding admission proper for impeachment purposes and not outweighed by its prejudicial impact), cert. denied, 488 U.S. 1004 (1989). "The question is whether the government examined the witness for the primary purpose of placing before the jury substantive evidence which is not otherwise admissible." United States v. Gomez-Gallardo, 915 F.2d 553, 555 (9th Cir.1990) (considering whether the government called witness to reveal through impeachment evidence that could not otherwise be brought before the jury).