A judicial officer may not preside over a matter in which a current client is either a party or a witness. A judicial officer may preside over a matter in which a former client is either a party or witness provided the subject matter of the litigation is unrelated to the matter on which the judicial officer represented the former client and there are no other circumstances which would call the judicial officer's impartiality into question. (CJC Canon 3(D)(1)(a))