static File installYouTubeDL() {
  File file = youTubeDLExecutable();
  if (!file.exists()) ret forceInstallYouTubeDL();
  pcall {
    if (urlSizeByHEAD(youTubeDL_URL()) != fileSize(file))
      ret forceInstallYouTubeDL();
  }
  ret file;
}