What is the test for challenging a decision by the Attorney General regarding scope certification under the Westfall Act?

"The Attorney General's decision regarding scope of employment certification [under the Westfall Act] is conclusive unless challenged. Accordingly, the party seeking review bears the burden of presenting evidence and disproving the Attorney General's decision to grant or deny scope of employment certification by a preponderance of the evidence." Green v. Hall, 8 F.3d 695, 698 (9th Cir. 1993) (per curiam) (citation and footnote omitted). "To rebut the [scope] certification . . . , a plaintiff must 'allege sufficient facts that, taken as true, would establish that the defendant's

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