static <A> L<A> lambdaMapLike varyList(IF1<A> varyElement, L<A> l) {
  if (empty(l)) null;
  int i = randomIndex(l);
  A varied = varyElement.get(l.get(i));
  if (varied == null) null;
  ret listWithSingleElementChanged(l, i, varied);
}