static <A> WithProbability<A> withProbability(double probability, A a) {
  ret WithProbability(probability, a);
}

static <A> L<WithProbability<A>> withProbability(IF1<A, Double> makeProbability, Iterable<A> l) {
  ret map(a -> withProbability(makeProbability.get(a), a), l);
}