They've never worked with portals. For OOP programers, portals can be coded in a remarkably small ammount of time.
Portals reuse all the basic polygon code .
When I realized that fact I went one step further and made a class for polygons and two derived classes, one for texture mapped polygons and one for portals. As I mentioned at the end of Constructive Solid Geometry it is possible to write an entire engine using nothing more than portals, but that's an excercise for another day. However, each red line is also the location of a portal. The portals are transparent polygons that describe the area in a model where each node touches another. Every portal contains information on the leaf nodes it is sandwiched between and so for the remainder of these pages I will reffer to portals by the nodes they touch.