public function data(string $slug = '', bool $by_slug = false)
    {
        if (!$slug) { // All data (i.e., for all slugs)?
            //
            if ($by_slug) { // All data keyed by slug?
                return $this->error_data ?: [$this->default_slug => null];
                //
            } else { // Data for the first slug.
                // NOTE: Different than `messages()`.
                $slug = $this->slug(); // First slug.
                return $this->error_data[$slug] ?? null;
            }
        } else {
            return $this->error_data[$slug] ?? null;
        }
    }