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.