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;

}