public function add($message, $scope = 'default')
    {
        $key = md5($scope.'~'.$message);
        $item = ['message' => $message, 'scope' => $scope];

        // don't add duplicates
        if (!array_key_exists($key, $this->messages)) {
            $this->messages[$key] = $item;
        }

        return $this;
    }