public function add(ExtensionField $extension)
    {
        $extendee = trim($extension->getExtendee(), '\\');
        $number   = $extension->getTag();

        if ( ! isset($this->extensions[$extendee])) {
            $this->extensions[$extendee] = [];
        }

        $this->extensions[$extendee][$number] = $extension;
    }