What is the proper cross-examination of a defendant who has never been convicted of any crime or offense?

After testifying on direct examination that he had never been convicted of any "crime or offense," the government's attorney was allowed to show on his cross-examination that he had, while in the army, twice been convicted of what the appellant called "a fight assault." We need not determine whether this would otherwise have been proper cross-examination, see United States v. Provoo, 2 Cir., 215 F.2d 531, 536, for it was permissible in contradiction of the appellant's previous testimony on the subject of his previous good conduct which he had himself put in issue.

The other asserted errors which were briefed and argued are too unsubstantial to require comment.