robot_localization state transition equation

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What is the state transition equation that is assumed in ekf_localization_node? More specifically, what is f in equation (1) of the paper?

I could potentially parse lines 241+ of the implementation, but that seems quite error prone and unnecessary.

My assumption, since it is used in the Husky robot, is that it assumes a differential drive robot, but I am unfamiliar with what these equations look like in 3-D. And I feel like this should really be documented somewhere.

Thanks!