REPUBLIC v. DISTRICT MAGISTRATE GRADE II, OSU; EX PARTE YAHAYA Criminal law and procedure—Trial—Summary proceedings—Plea of not guilty— Act 30, s. 172 setting out mandatory procedure to be followed where accused pleading not guilty—Accused pleading not guilty to charge—Whether magistrate justified in suo motu changing plea to guilty after hearing explanation of accused—Criminal Procedure Code, 1960 (Act 30), s. 172.