If at any trial by a military court beyond the borders of the Republic a competent but not compelled witness gives evidence but refuses to answer any question put to him by the court, or by the defence if he has been called by the prosecution, or by the prosecutor if he has been called by the defence, to which question he could be bound in law to reply if he were a witness at such a trial in the Republic, the court shall, if satisfied that the answer to the question is material order the witness to stand down and strike the whole of his evidence from the record of the proceedings.