CSG level editor

First i wanted to tell that i am writting a level editor(uses CSG). Ive run in to some problem with it recently... Basic idea is that level geometry will be substarcted from the Solid Space. Ok, first i ve got brushes(for term brush i understand a convex solid), which face normal point inside solid, lets call them "SUB" brushes. Those brushes substract mass from level Solid Space. I can merge them by removing face fragments that are inside, creating rooms and other empty areas. Next i wanted to add something inside empty areas, such as creating boxes and other solid ''things''. I was trying to use brush "ADD", this brush differs from SUB with face normals pointing out of a solid. But ''CSGing'' ADD brushes with SUB ones producess a one big mess. I known that clipping ADD brush faces with SUB must retain fragments that are outside SUB but this didnt work. Could somebody implemented such a level editor? Could somebody tell me how program must act when clipping SUB with ADD, and ADD with SUB? What about clipping order? TIA regards, J.Black