// discriminator: func(A) -> bool
static <A, B extends Collection<A>> Scorer scoreDiscriminator(O discriminator, Pair<B> p) {
  new Scorer scorer;
  for (A a : unnull(p.a))
    scorer.add(a, isTrue_callF(discriminator, a));
  for (A a : unnull(p.b))
    scorer.add(a, !isTrue_callF(discriminator, a));
  ret scorer;
}