What is the scope of the Fifth Amendment privilege waiver when a defendant testifies at trial?

"A defendant who testifies at trial waives his Fifth Amendment privilege and may be cross-examined on matters made relevant by his direct testimony.... The scope of the defendant's waiver is coextensive with the scope of the relevant cross-examination." United States v. Black, 767 F.2d 1334, 1341 (9th Cir.), cert. denied, 474 U.S. 1022 (1985) (citations omitted). On cross-examination, the government may ask questions which are " 'reasonably related' to the subjects covered by the defendant's testimony." United States v. Hearst, 563 F.2d 1331, 1340 (9th Cir.1977), cert. denied, 435 U.S. 1000 (1978).