Function has no implementation

I was looking though code and found, that this function have no implementation, despite having a big comment section above it. Why is that?

https://github.com/Scrawk/Brunetons-Improved-Atmospheric-Scattering/blob/b360e2cd1ebf32b0438ae7688aa30e9a06c6df09/Assets/BrunetonsImprovedAtmosphere/Shaders/ScatteringFunctions.cginc#L550