This is an example of a program that uses librt to shoot a single ray at some
        geometry in a geometry (.g) database.
        librt is a big library with lots of code, but basically all that the programmer
        needs to do is include `raytrace.h` and specify the following:
          a_rt_i    a raytrace instance, from 'rt_dirbuild()'
          a_ray     ray origin and direction
          a_hit     function to be called if ray hits something
          a_miss    function to be called if ray misses everything
        rtexample defines simple functions for a_hit and a_miss which simply
        output information about hit region (if hit) and log misses. Applications will
        typically define more complex tasks to fo if the ray hits, such as describing a
        reflected ray.