static <A> L<A> withoutNulls(Iterable<A> l) {
  if (l cast L)
    if (!containsNulls(l)) ret l;
  new L<A> l2;
  for (A a : l)
    if (a != null)
      l2.add(a);
  ret l2;
}

static <A, B> Map<A, B> withoutNulls(Map<A, B> map) {
  Map<A, B> map2 = similarEmptyMap(map);
  for (A a : keys(map))
    if (a != null) {
      B b = map.get(a);
      if (b != null)
        map2.put(a, b);
    }
  ret map2;
}

static <A> L<A> withoutNulls(A[] l) {
  new L<A> l2;
  if (l != null) for (A a : l)
    if (a != null)
      l2.add(a);
  ret l2;
}