static L<IntPair> overlappingIntPairs(L<Int> l) {
  new L<IntPair> out;
  Iterator<Int> it = iterator(l);
  if (!it.hasNext()) ret out;
  int a = it.next();
  while (it.hasNext()) {
    int b = it.next();
    out.add(IntPair(a, b));
    a = b;
  }
  ret out;
}