public function getFailureNames() : array
    {
        if ($this->failures) {
            $keys = array_keys($this->failures);
            $net = [];
            foreach ($keys as $key) {
                if ($key && !ctype_digit('' . $key)) {
                    $net[] = $key;
                }
            }
            return $net;
        }
        return [];
    }