When May a Judge Do Something that's Required?

Love this one.  The Wisconsin statute provides:

If a person complains to a judge that he or she has reason to believe that a crime has been committed within his or her jurisdiction, the judge shall examine the complainant under oath and any witnesses produced by him or her and may, and at the request of the district attorney shall, subpoena and examine other witnesses to ascertain whether a crime has been committed and by whom committed. The extent to which the judge may proceed in the examination is within the judge's discretion. . . .

Does the judge HAVE to examine all the witnesses?  No, says the Wisconsin Supreme Court in a case decided here, Wisconsin v. Madden, __ N.W.2d __ (Wis. June 2009).