code: function(nodes, controller) {
          var counter = 0, len = nodes.length, nodeSelected = {};
          var complete = function() { controller.onComplete(); };
          var viz = this.viz;
          if(len == 0) complete();
          for(var i=0; i<len; i++) {
              nodeSelected[nodes[i].id] = nodes[i];
              controller.request(nodes[i].id, nodes[i]._level, {
                  onComplete: function(nodeId, data) {
                      if(data && data.children) {
                          data.id = nodeId;
                          viz.op.sum(data, { type: 'nothing' });
                      }
                      if(++counter == len) {
                          viz.graph.computeLevels(viz.root, 0);
                          complete();
                      }
                  }
              });
          }
      }
docs: /*
  
  Calls the request method on the controller to request a subtree for each node.