About rotation and self.reference_target_orn

[Xianqi-Zhang]

Hi, Thanks for your sharing. I have a question about rotation. In controllers.ik.py Line-393

# Update reference target orientation
self.reference_target_orn = T.quat_multiply(self.reference_target_orn, base_orn_in_world)

I am very confused about the fact that self.reference_target_orn needs to be continuously multiplied by base_orn_in_world and then saved, what is the purpose of doing this? And the quaternions here do not use $qpq^{-1}$ to calculate rotation as written in many tutorials, why is this? Is there any book or tutorial that I can refer to?

Thank you for your any reply.

Best regards.