179. On the application of the prosecutor, the judge, on such conditions as he determines and if he is satisfied that no injustice will result therefrom, shall allow him to amend a count so as to add a detail or correct an irregularity, and in particular to include in it, in express terms, an essential element of the offence. In no case may the judge allow one defendant to be substituted for another or one offence to be substituted for another.