What are the requirements of a federal district court under section 851(b) of the Criminal Code to notify a defendant of their prior conviction before sentencing?

Then after conviction but before imposing the sentence, the district court "shall ... inquire of the [offender] ... whether he affirms or denies [the prior conviction] ..., and shall inform him that any challenge to a prior conviction" is waived if not made before sentencing. Id. 851(b) (emphasis added). "We require strict compliance with the procedural aspects of section 851(b). The 851(b) colloquy is not merely a procedural requirement. It serves a functional purpose to place the procedural onus on the district court to ensure defendants are fully aware of their rights." United States v. Rodriguez, 851 F.3d 931, 946 (9th Cir. 2017) (internal quotation marks and citation omitted).