static S ngramScoreBySet(int n, S text, Set<S> set1, Set<S> set2, S a, S b) {
  int score = 0;
  for (S s : ngrams(n, text))
    score += zeroOne(set1.contains(s))-zeroOne(set2.contains(s));
  ret score > 0 ? a : score < 0 ? b : null;
}