How to create a parent layer?

A Swival Ring unit has 4 parts and each part has been assigned by a different layer as the attached file shown.
I want to create a parent layer, namely swival ring and to assign those 4 different layers to be the sublayers of the layer swival ring.
Is it possible?
Swival Ring.3dm (6.1 MB)

Create a new layer, select the four layers and drag them into the newly created layer.

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