static <A> L<Pair<A>> listToOverlappingPairs(L<A> l) {
  new L<Pair<A>> out;
  Iterator<A> it = iterator(l);
  if (!it.hasNext()) ret out;
  A a = it.next();
  while (it.hasNext()) {
    A b = it.next();
    addPair(out, a, b);
    a = b;
  }
  ret out;
}