[Therion] Can't change default l_wall style with metapost

Hi, I have changed wall subtypes before in my layout. e.g. Using Bruce Mutton's code metapost def l_wall_unsurveyed (expr P) = T:=identity; pickup PenC; thdraw P dashed evenly scaled (2*optical_zoom); enddef; endcode However, I can't get this working on the default l_wall type. Any changes, including using the same style as in the above l_wall_unsurveyed example get ignored. e.g. code metapost def l_wall (expr P) = T:=identity; pickup PenC; thdraw P dashed evenly scaled (2*optical_zoom); enddef; endcode I'm not sure what I'm missing. I'm first trying to prove I can override this, and then working on a wall style with a shadow or external cross-hatching. Thanks, Paul