>You may use any line symbol as "wall" (in fact as a scrap outline)  … all you need to do is to add "-outline out" option to it. 

At first I thought this is a revolutionary idea!

Then I searched the Therion Book and discovered that it has long been described in the chapters ‘How to draw maps’ and ‘Scrap’ on pg 21.

Thanks for pointing it out, I must try it out.

Bruce