// pred: func(int) -> bool
static <A> Pair<L<A>> mapLike filterAntiFilterListByPredicateOnIndex(O pred, L<A> l) {
  new L<A> yes;
  new L<A> no;
  int n = l(l);
  for i to n:
    (isTrue_callF(pred, i) ? yes : no).add(l.get(i));
  ret pair(yes, no);
}

Began life as a copy of #1024711