static <A> A collectionEntryAtIndex(Collection<A> l, int i) {
  if (l cast L) ret (A) get(l, i);
  if (i < 0 || i >= l(l)) null;
  Iterator<A> it = iterator(l);
  while (i-- > 0) if (!it.hasNext()) null; else it.next();
  ret it.hasNext() ? it.next() : null;
}