the relevant provisions of Rules 39 are in the following terms: "Rule 39 . . (2) An officer is disqualified for serving on a general or district court martial if he: (a) is an officer who convened the court; or (b) is the prosecutor or a witness for the prosecution; or (c) investigated the charges before trial, or took down the summary of evidence, or was a member of a court of inquiry respecting the matters on which the charges against the accused are founded, or was the squadron, battery, company, or other commander, who made preliminary inquiry into the case, or was a member of a previous court martial which tried the accused in respect of the same offence; or (d) is the commanding officer of the accused, or of the corps to which the accused belongs; or (e) has a personal interest in the case." Rule 39(2) provides that an officer who is the Commanding Officer of the accused or of the corps to which the accused belongs or who is an officer who convened the court or who is the prosecutor or a witness for the prosecution and who has a personal interest in the case, is not eligible for serving on a general or district Court Martial.