static <A> Chain<A> toChain(Iterable<A> l) {
  Iterator<A> it = iterator(l);
  if (!it.hasNext()) null;
  Chain<A> c = new(it.next());
  Chain<A> last = c;
  int size = 1;
  while (it.hasNext()) {
    ++size;
    Chain x = new(it.next());
    last.next = x;
    last = x;
  }
  
  // update sizes and return
  
  Chain<A> x = c;
  while (x != null) {
    x.size = size--;
    x = x.next;
  }
  ret c;
}