docs: function(csg) {
              var newpolygons = this.polygons.concat(csg.polygons);
              var result = CSG.fromPolygons(newpolygons);
              result.properties = this.properties._merge(csg.properties);
              result.isCanonicalized = this.isCanonicalized && csg.isCanonicalized;
              result.isRetesselated = this.isRetesselated && csg.isRetesselated;
              return result;
          }