[Therion] changing wall symbols

Hello On some scales the wall:debris and blocks does not work very well. So I thought that I would change the symbol. So I have been using code metapost let p_gradient = p_entrance_UIS; Which works fine. so copying that I tried code metapost let l_wall_debris = l_wall_UIS; Which had no effect. With a bit more playing I discovered I could not change any wall things like code metapost let l_wall = l_floorstep_UIS; further experiments showed I could not change any line symbol code metapost let l_overhang = l_wall_UIS; one problem is that an error is not generated if you make a typo so I cannot tell if I have the right identifiers. What am I doing wrong? Andrew