We need not discuss the question whether the Commonwealth would be barred from prosecuting a person for an offence by the fact that the attorney general, district attorney, or other prosecuting officer had promised such person immunity, upon calling him as a witness in the prosecution of another person for the same offence, or one connected with it. Such question is not raised in the case at bar, because there is nothing to show that the city marshal made any promise to the defendant, express or implied, that he should be protected from prosecution. Commonwealth v. Brown, 103 Mass. 422. Commonwealth v. Denehy, 103 Mass. 424, note.

Exceptions overruled.