static <A> L<A> uniquifyList(Collection<A> l) {
  if (l == null) null;
  if (l(l) < 2) ret asList(l);
  new HashSet<A> set;
  new L<A> out;
  for (A a : l)
    if (set.add(a))
      out.add(a);
  ret out;
}