remove extra volume

demos/component8_in_a_box.geo

@@ -22,9 +22,10 @@ Line(10003) = {10002, 10001};
 Line(10004) = {10001, 10004};
 Line Loop(10005) = {10002, 10003, 10004, 10001};
 Plane Surface(10006) = {10005};
-Extrude {0, 0, (zmax - zmin) + 2 * l} {
+tmp[] = Extrude {0, 0, (zmax - zmin) + 2 * l} {
   Surface{10006};
-}
+};
+Delete { Volume{tmp[1]}; }
 
 Surface Loop(10029) = {10027,10006,10015,10019,10023,10028};
 Surface Loop(10030) = {1,2,3,4,21,15,16,17,18,19,20,5,6,7,8,9,10,11,12,13,14};