as great as that of the fifth amendment privilege that the witness must forego. See Kastigar v. United States, 406 US. at 449, 92 S.Ct. at 1659; Counselman v. Hitchcock 142 U.S. 547, 564, 586-87, 12 S.Ct. 195, 198, 206, 95 LEd. 1110 (1892). As such, in deciding the scope of a grant of immunity the Sup