code: function(node, bag) {
      if (!bag) {
        return 1;
      }
      var id = _nodeUID(node);
      if (!bag[id]) {
        return bag[id] = 1;
      }
      return 0;
    }
docs: determine if a node in is unique in a 'bag'. In this case we don't want to flatten a list of unique items, but rather just tell if the item in question is already in the bag. Normally we'd just use hash lookup to do this for us but IE's DOM is busted so we can't really count on that. On the upside, it gives us a built in unique ID function.