static <A> A collectionGet(Collection<A> c, int idx) {
  if (c == null || idx < 0 || idx >= l(c)) null;
  if (c instanceof L) ret listGet((L<A>) c, idx);
  Iterator<A> it = c.iterator();
  for (int i = 0; i < idx; i++) if (it.hasNext()) it.next(); else null;
  ret it.hasNext() ? it.next() : null;
}