public static function accept($key='type',$choices='') {
    if (null === static::$accepts) static::$accepts = [
      'type'     => new Negotiation(isset($_SERVER['HTTP_ACCEPT'])          ? $_SERVER['HTTP_ACCEPT']          : ''),
      'language' => new Negotiation(isset($_SERVER['HTTP_ACCEPT_LANGUAGE']) ? $_SERVER['HTTP_ACCEPT_LANGUAGE'] : ''),
      'encoding' => new Negotiation(isset($_SERVER['HTTP_ACCEPT_ENCODING']) ? $_SERVER['HTTP_ACCEPT_ENCODING'] : ''),
      'charset'  => new Negotiation(isset($_SERVER['HTTP_ACCEPT_CHARSET'])  ? $_SERVER['HTTP_ACCEPT_CHARSET']  : ''),
    ];
    return empty(static::$accepts[$key])
      ? false
      : ( empty($choices)
           ? static::$accepts[$key]->preferred()
           : static::$accepts[$key]->best($choices)
      );
  }