public function prepend($key, $value) {
    do {
      $result = $this->get($key, null, $cas_token);
      if (!$result || !is_string($result)) {
        $this->result_code = self::RES_NOTSTORED;
        return false;
      }

      $result = $value . $result;
      $result = $this->cas($cas_token, $key, $result);
    } while (!$result && $this->result_code == self::RES_DATA_EXISTS);

    $this->result_code = $result ? self::RES_SUCCESS : self::RES_NOTSTORED;
    return $result;
  }