// returns empty first and empty last
static <A> LL<A> splitAtListElement_multi(L<A> l, A splitter) {
  l = unnull(l);
  new LL<A> parts;
  int i = 0;
  while true {
    int j = indexOf(l, splitter, i);
    if (j < 0) {
      parts.add(subList(l, i));
      break;
    }
    parts.add(subList(l, i, j));
    i = j+1;
    while (i < l(l) && eq(l.get(i), splitter)) ++i;
  }
  ret parts;
}