sS javaMail_messageContentToText(O content) {
  if (isString(content)) ret content;
  if (content instanceof Multipart) {
    Multipart mp = cast content;
    int n = mp.getCount();
    new L<S> l;
    for i to n: {
      Part part = mp.getBodyPart(i);
      S type = part.getContentType();
      O partContent = part.getContent();
      l.add(type + " / " + className(partContent));
      l.add(indentx(javaMail_messageContentToText(partContent)));
    }
    ret joinWithEmptyLines(l);
  }
  ret "Unknown content type: " + className(content);
}