Is a defendant entitled to a jury instruction on any legal defense to the charge against him which has some foundation in the evidence?

"[A] criminal defendant is entitled to have a jury instruction on any legal defense to the charge against him which has some foundation in the evidence." United States v. Chen, 933 F.2d 793, 796 (9th Cir.1991) (citation omitted). However, "[s]o long as the instructions fairly and adequately cover the issues presented, the judge's formulation of those instructions or choice of language is a matter of discretion." Id. (citations and quotation marks omitted). Appellate courts review district court instructions as a whole and "consider how they will reasonably be understood by the jury in the context of the whole trial." Id. (citations and quotation marks omitted).