code: function(n) {
      var la = this.lookahead(n).type;
      switch (la) {
        case 'for':
          return this.bracketed;
        case '[':
          this.bracketed = true;
          return true;
        case ']':
          this.bracketed = false;
          return true;
        default:
          return ~selectorTokens.indexOf(la);
      }
    }
docs: Check if the token at `n` is a valid selector token.
  
  @param {Number} n
  @return {Boolean}
  @api private