code: function group() {
      var ret = this.seq(
          this.stringf('('),
          this.manyf(c_wsp),
          alternation,
          this.manyf(c_wsp),
          this.stringf(')'));
      return new ast.Group(ret[3]);
  }
docs: group          =  "(" *c-wsp alternation *c-wsp ")"