protected function buildIdAttr($eltId = null, bool $errorIfExists = false)
    {
        if ($errorIfExists && isset($this->attributes['id'])) {
            throw new \Osf\Exception\ArchException('Id element already set');
        }
        if ($eltId === null) {
            $eltId = 'e' . self::$eltIdCount++;
        }
        $this->setAttribute('id', $eltId);
        return $this;
    }