Whether courts should determine a controlled substance offense under U.S.S.G. § 4B1.2(b) based on the substances controlled at the time of the prior conviction or at the time of federal sentencing

1. When defining a controlled substance offense under U.S.S.G. § 4B1.2(b) do courts look to the substances that were controlled on the date of the prior conviction or to the substances controlled on the date of federal sentencing?