static <A> L<Int> indicesOf(L<A> l, A a) {
  if (l == null) null;
  
  ifclass IContentsIndexedList
  if (l cast IContentsIndexedList)
    ret intArrayToList(l.indicesOf(a));
  endif
  ifclass IContentsIndexedList2
  if (l cast IContentsIndexedList2)
    ret map(h -> h/HasIndex.idx, l.indicesOf_treeSetOfHasIndex(a));
  endif
  
  new L<Int> x;
  for (int i = 0; i < l(l); i++)
    if (eq(l.get(i), a))
      x.add(i);
  ret x;
}

static <A> L<Int> indicesOf(S s, S x) {
  int i = -1;
  new L<Int> out;
  while ((i = indexOf(s, x, i+1)) >= 0)
    out.add(i);
  ret out;
}

Began life as a copy of #1004477