What is the test for "substantial interference" with a witness' choice not to testify?

Where there is a claim that the court or the prosecutor interfered with the witness' voluntary choice whether to testify, only will a "substantial" interference with a witnesses' decision implicate the defendant's rights. United States v. Vavages, 151 F.3d 1185, 1189 (9th Cir. 1998). If a judge's advisal to a witness is threatening or coercive, the defendant is deprived of due process if the witness is "effectively driven off the stand." Webb v. Texas, 409 U.S. 95, 98 (1972). Where the prosecutor's

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