An attorney is representing a defendant on trial for armed robbery of a liquor store. the defendant tells the attorney in confidence that at the time in question, he was sitting at home watching television with his aged mother, and that his mother can confirm his alibi. the attorney interviews the mother, who solemnly confirms the defendant's story. after talking with her, the attorney strongly suspects that she is lying to protect the defendant. the attorney does not know for sure that the defendant and his mother are lying, but every instinct tells him that they are. the attorney has warned both of them about the dangers of perjury, but both have insisted that they want to testify to the alibi at trial. may the attorney call the defendant, or his mother, or both, as trial witnesses?