// defaults to median when idx isn't given
static <A> A ithSmallest(L<A> l, int idx default l(l)/2) {
  int n = l(l);
  if (n <= 80)
    ret get(sorted(l), idx);
  int cols = (n+4)/5;
  new L<A> medians;
  for (int i = 0; i < cols; i++)
    medians.add(ithSmallest(subList(l, i*5, 5)));
    
  // median of medians
  A mom = ithSmallest(medians);
  
  L<A> setA = filter(l, a -> lessThan(a, mom));
  L<A> setB = filter(l, a -> greaterThan(a, mom));
  
  int nA = l(setA);
  if (idx < nA)
    ret ithSmallest(setA, idx);
  if (idx == nA)
    ret mom;
  ret ithSmallest(setB, idx-nA-1);
}