24       tree:       false  property
59     if(drop.tree) {
607       tree:        false,  property
660       tree:        options.tree,  property
679     if(options.dropOnEmpty || options.tree) {
691       if(options.tree) e.treeNode = element;
695     if(options.tree) {
714       element, options.only, options.tree ? true : false, options.tag);
719       element, options.only, options.tree ? true : false, options.treeTag);
725     if(overlap > .33 && overlap < .66 && Sortable.options(dropon).tree) {
838   tree: function(element) {  method
904     if (options.tree) {
905       return Sortable.tree(element, arguments[1]).children.map( function (item) {