docs: def instance(self):
          """
          Retrieve the single (expected) instance of this 'Part' (of `Category.MODEL`) as a 'Part'.
  
          See :func:`Part.instances()` method for documentation.
  
          :return: :class:`Part` with category `INSTANCE`
          :raises NotFoundError: if the instance does not exist
          :raises MultipleFoundError: if there are more than a single instance returned
          """
          instances_list = list(self.instances())
          if len(instances_list) == 1:
              return instances_list[0]
          elif len(instances_list) > 1:
              raise MultipleFoundError("Part {} has more than a single instance. "
                                       "Use the `Part.instances()` method".format(self.name))
          else:
              raise NotFoundError("Part {} has no instance".format(self.name))