code: def _(mcs, cls_name="Object", with_meta=None):
          
          if with_meta is not None:
              class EXType(with_meta, mcs):
                  _cls_name = cls_name
                  _base_classes = []
                  _generated_class = None
          else:
              class EXType(mcs):
                  _cls_name = cls_name
                  _base_classes = []
                  _generated_class = None
  
          return EXType
docs: Method to generate real metaclass to be used::
  
                  mc = ExtensibleType._("MyClass")  # note this line
                  @six.add_metaclass(mc)
                  class MyClassBase(object):
                      pass
  
              :param str cls_name: name of generated class
              :param class with_meta: Mix aditional metaclass in.
                                      (default: None)
              :return: specific metaclass to track new inheritance tree