<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def visible_object_layers(self):
 """ This must return layer objects This is not required for custom data formats. :return: Sequence of pytmx object layers/groups """

        return (layer for layer in self.tmx.visible_layers
                if isinstance(layer, pytmx.TiledObjectGroup))