code: function(nodes, controller) {
          var viz = this.viz;
          var that = this;
  
          nodes = this.prepare(nodes);
          this.animation.setOptions($.merge(controller, {
              $animating: false,
              compute: function(delta) {
                if(delta == 1) delta = 0.99;
                that.plotStep(1 - delta, controller, this.$animating);
                this.$animating = 'contract';
              },
              
              complete: function() {
                  that.hide(nodes, controller);
              }       
          })).start();
      }
docs: /*
  
  Collapses group of nodes.