What is the standard of review under the doctrine of plain error review?

^1. Where, as here, "the appeal presents a pure question of law and there is no prejudice to the opposing party that resulted from a defendant's failure to object," this court is "not limited to a plain error standard of review." United States v. Joseph, 716 F.3d 1273, 1276 n.4 (9th Cir. 2013) (alterations and internal quotation marks omitted). Because Rodriguez agrees that plain error review applies, we express no opinion on whether, under de novo review, we would agree with the district court's interpretation.