function __traverse_items(item, callback)
{
   callback(item);
   for (var i = 0; i < item.children.length; i++)
   {  __traverse_items(item.children[i], callback);
   }
}

function traverse_items(item_id, callback)
{
   root = all_items_by_id[item_id];
   __traverse_items(root, callback);
}