I did not post it before because the definition was extremely unorganized and very "touchy".
I have since rebuilt the definition to be compatible with GH v.0.6. Also, the definition no longer relies on solid booleans to create the rib notches (I have found solid booleans to be extremely unreliable in Grasshopper).
Anyway... now available by popular demand:

Note: I make no guarantees for how well the definition works. Sometimes it has problems when a vertical rib intersects a surface seam... Suggestions welcome!