What is the test for denying a criminal conviction under section 851(c)(1) of the Criminal Code?

See generally Almendarez-Torres v. United States, 523 U.S. 224 (1998).⁵ While 851(c)(1) requires the government to prove contested facts relating to the prior felony beyond a reasonable doubt, that burden is triggered only where the defendant denies the prior felony and submits a written response raising a factual issue. 21 U.S.C. 851(c)(1). The statute, moreover, specifically requires the court to warn the defendant that failure to challenge the fact of the prior conviction before sentencing waives any such challenge thereafter. Id. 851(b). We therefore do not assume that ambiguous answers or refusals to answer a 851 inquiry should be treated as tantamount to a denial.