docs: function pathIndex() {
    var args = mlutil.asArray.apply(null, arguments);
    switch(args.length) {
    case 0:
      throw new Error('missing path for path index identifier');
    case 1:
      return new PathIndexDef(new PathDef(args[0]));
    case 2:
      return new PathIndexDef(new PathDef(args[0], args[1]));
    default:
      throw new Error('too many arguments for path index identifier: '+args.length);
    }
  }