Christian,
Thanks Christian. How one is meant to know this I have no idea. Maybe it was meant to be in the missing 3rd chapter of the Extending manual.
No, one is meant to search the source. There are many scheme functions in the source that are used internally, but are not documented externally. Advanced users will search the source code for these functions. In the case of the LSR, you have examples of their use. So you know the name of the function, and you can search the source to find the code.
If you are working on a machine (e.g. windows) that doesn't give you ready access to the source files, you can always go to the repository at GitLab or Savannah and search the source online.
Best wishes,
Carl