private function addToFrontmatter($matter)
    {
        if (empty($matter) || !is_array($matter)) {
            return $this;
        }

        if (is_null($this->current->frontmatter)) {
            $this->current->frontmatter = [];
        }

        $this->current->frontmatter = array_merge($this->current->frontmatter, $matter);
        return $this;
    }