protected function deleteExistingSitemaps($targetDir)
    {
        foreach ($this->urlsets as $urlset) {
            $basename = basename($urlset->getLoc());
            if (preg_match('/(.*)_[\d]+\.xml(?:\.gz)?$/', $basename)) {
                continue; // skip numbered files
            }
            // pattern is base name of sitemap file (with .xml cut) optionally followed by _X for numbered files
            $basename = preg_replace('/\.xml(?:\.gz)?$/', '', $basename); // cut .xml|.xml.gz
            $pattern = '/' . preg_quote($basename, '/') . '(_\d+)?\.xml(?:\.gz)?$/';
            foreach (Finder::create()->in($targetDir)->depth(0)->name($pattern)->files() as $file) {
                $this->filesystem->remove($file);
            }
        }
    }