code: def _set_signed_in(self):
          
          names = [
              controller.get_user_name(user, full_name=CONFIG['FULL_USER_NAMES'])
              for user in controller.signed_in_users()
          ]
          self.signed_in.set('\n'.join(sorted(names)))
docs: Populate the signed_in list with the names of currently
          signed in users.