final public static function registerTagHandler($tag, $handler)
    {
        $tag = trim((string)$tag);

        if (null === $handler) {
            unset(self::$tagHandlerMappings[$tag]);
            return true;
        }

        if ('' !== $tag
            && class_exists($handler, true)
            && is_subclass_of($handler, __CLASS__)
            && !strpos($tag, '\\') //Accept no slash, and 1st slash at offset 0.
        ) {
            self::$tagHandlerMappings[$tag] = $handler;
            return true;
        }

        return false;
    }