static Substring joinSubstringObjects(Iterable<Substring> l) {
  Iterator<Substring> it = iterator(l);
  if (!it.hasNext()) null;
  Substring s = it.next();
  while (it.hasNext()) {
    Substring b = it.next();
    if (s.s != b.s) fail("Substrings have different base string");
    if (s.endIndex() != b.startIndex()) fail("Substrings don't fit: " + s.range() + " / " + b.range());
    s = Substring(s.s, s.startIndex(), s.length()+b.length());
  }
  ret s;
}