public function expectExplicit($expectedTag = null): ExplicitTagging
    {
        $el = $this;
        if (!$el instanceof ExplicitTagging) {
            throw new \UnexpectedValueException(
                "Element doesn't implement explicit tagging.");
        }
        if (isset($expectedTag)) {
            $el->expectTagged($expectedTag);
        }
        return $el;
    }