// 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(isTrue_callF(discriminator, a), a);
  for (A a : unnull(p.b))
    scorer.add(!isTrue_callF(discriminator, a), a);
  ret scorer;
}