In Murphy v. Waterfront Comm'n, 378 U.S. 52, 79, 12 L. Ed. 2d 678, 84 S. Ct. 1594 (1964), the Supreme Court explained why that rule-- that state and federal witnesses, compelled to testify in the face of the assertion of a privilege, must be granted both state and federal immunity-is necessary. Most, if not all of the policies and purposes of the constitutional privilege against self-incrimination are defeated if a witness can be "whipsawed" into incriminating himself under both state and federal law but only one of them provides the witness immunity while the other is free to prosecute based upon his immunized testimony. Id. at 55-56.