Pete hopes there's a clean way to do this. Perhaps we're doomed to ugly code. Want a scene with an analytic solution. Suppose one is in a scene, sends out a viewing ray, bounce once into ray L' from point x1' with refl. Notice there's no geometry in there.
Geometry is implicitly in there.
Brute force RT'er has to be pretty dumb not to get this answer. This is a good debugging case for this particular method. Might add an error estimate on the end. Ray comes in, send reflected ray with probability p, terminate with probability 1-p.