In the pair_style polymorphic command documentation, I understand that users can modify the six functions (U, V, F, P, W, G) to create their own analytical or tabular form of the potentials. I am looking to modify to a Stillenger-Weber like three-body term. Where do I modify these six functions for the analytical form? If I understand correctly, the examples in the "potential" folder are for the tabulated form. Is there an example for the analytical form?