Agrand jury was investigating a bank robbery. the only information known to the prosecutor was a rumor that a certain ex-convict might have been involved. the grand jury subpoenaed the ex-convict. he refused to answer questions about the robbery and was granted use and derivative use immunity. he then testified that he and a friend had robbed the bank. the grand jury indicted both the ex-convict and his friend for the bank robbery. the prosecutor permitted the friend to enter a plea to a lesser offense in exchange for the friend’s agreement to testify against the ex-convict. the prosecutor had no evidence as to the identity of the robbers except the testimony of the friend and the ex-convict. at the ex-convict’s trial, should his objection to his friend’s being permitted to testify be sustained?