code: function(node, collapsedChildren){
      var children = node.data('collapsedChildren');
      var i;
      for (i=0; i < children.length; i++){
        if (children[i].data('collapsedChildren')){
          collapsedChildren = collapsedChildren.union(this.getCollapsedChildrenRecursively(children[i], collapsedChildren));
        }
        collapsedChildren = collapsedChildren.union(children[i]);
      }
      return collapsedChildren;
    }
docs: Get all collapsed children - including nested ones
  @param node : a collapsed node
  @param collapsedChildren : a collection to store the result
  @return : collapsed children