code: function (plane, coplanarfrontnodes, coplanarbacknodes, frontnodes, backnodes) {
              var polygon = this.polygon;
              if (polygon) {
                  var bound = polygon.boundingSphere();
                  var sphereradius = bound[1] + 1e-4;
                  var planenormal = plane.normal;
                  var spherecenter = bound[0];
                  var d = planenormal.dot(spherecenter) - plane.w;
                  if (d > sphereradius) {
                      frontnodes.push(this);
                  } else if (d < -sphereradius) {
                      backnodes.push(this);
                  } else {
                      var splitresult = plane.splitPolygon(polygon);
                      switch (splitresult.type) {
                          case 0:
                              // coplanar front:
                              coplanarfrontnodes.push(this);
                              break;
  
                          case 1:
                              // coplanar back:
                              coplanarbacknodes.push(this);
                              break;
  
                          case 2:
                              // front:
                              frontnodes.push(this);
                              break;
  
                          case 3:
                              // back:
                              backnodes.push(this);
                              break;
  
                          case 4:
                              // spanning:
                              if (splitresult.front) {
                                  var frontnode = this.addChild(splitresult.front);
                                  frontnodes.push(frontnode);
                              }
                              if (splitresult.back) {
                                  var backnode = this.addChild(splitresult.back);
                                  backnodes.push(backnode);
                              }
                              break;
                      }
                  }
              }
          }
docs: only to be called for nodes with no children