static S unsign(S s) {
  try {
    L<S> lines = toLines(s);
    S sigLine = lines.get(lines.size()-1);
    if (!sigLine.startsWith("sig: "))
      ret s;
    ret fromLines(lines.subList(0, lines.size()-2));
  } catch (RuntimeException e) {
    ret s;
  }
}