How do I merge two URDFs?

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Hello!
I would like to merge robotic arm URDF with selected gripper URDF using your library. How can I do that?

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Imagine I have this code:

arm = URDF.load(arm_urdf_path)
gripper = URDF.load(gripper_urdf_path)

# How to attach gripper as an end effector to the arm?

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@mmatl