// possible binary legacy signature
static <A> L<Pair<A>> listToPairs(L<A> l) {
  ret listToPairs((Iterable<A>) l);
}

static <A> L<Pair<A>> listToPairs(Iterable<A> or A[] l) {
  new LPair<A> out;
  Iterator<A> it = iterator(l);
  while ping (it.hasNext()) {
    A a = it.next();
    if (!it.hasNext()) break;
    A b = it.next();
    addPair(out, a, b);
  } 
  ret out;
}