docs: function(filterFunc) {
      filterFunc = filterFunc || goog.functions.TRUE;
      return function(root, ret, bag) {
        var te, x = 0, tret = root[childNodesName];
        while (te = tret[x++]) {
          if (
            simpleNodeTest(te) &&
            (!bag || _isUnique(te, bag)) &&
            (filterFunc(te, x))
          ) {
            ret.push(te);
          }
        }
        return ret;
      };
    }