Is there any example about how to add user-defined macros or functions in cel-cpp

We use cel-cpp to write some rules for trigger the action when some condition matched. For some sophisticated rules，we need use some user-defined macros or functions, for example: compute the 3D IOU for two 3D objects(proto message), compute the included angle of two 3D vector and so on.

After search the codebase, we can not find examples about how to add user-defined macros or functions in cel-cpp.