What is the burden of proving that a defendant has been convicted of at least two serious violent felonies under S 3559(c)(2)(F) of the three-strikes law?

that a defendant was convicted of at least two prior offenses that qualify as "serious violent felonies" under S 3559(c)(2)(F). Defendant asserts that the statute fails to tell us who must prove the existence of the prior strikes. We disagree. The structure of the statute shows that this is the government's burden because, in the absence of proof of qualifying prior convictions, a defendant receives the standard, lesser sentence and thus benefits from the absence of proof. Additionally, the purpose of the statute is consonant with requiring the government to prove the qualifying prior convictions, because they serve to enhance the sentence. See United States v. Young, 33 F.3d 31, 32 (9th Cir. 1994) ("The Government bears the burden of proving factors enhancing a sentence by a preponderance of the evidence."); United States v. Oberle, 136 F.3d 1414, 1423-24 (10th Cir.) (holding that the government bears the burden of proving the prior strikes under the three-strikes law), cert. denied, 119 S. Ct. 197 (1998).