@classmethod
    def register(cls):
        pass

    @classmethod
    def unregister(cls):
        pass

class CyclesRenderSettings(bpy.types.PropertyGroup):
    @classmethod
    def register(cls):
        bpy.types.Scene.cycles = PointerProperty(
                name="Cycles Render Settings",
                description="Cycles render settings",
                type=cls,
                )
        cls.device = EnumProperty(
                name="Device",
                description="Device to use for rendering",
                items=enum_devices,
                default='CPU',
                )
        # ... and so on adding properties to cls

    @classmethod
    def unregister(cls):
        del bpy.types.Scene.cycles