sS ai_combineNGramScorers_noDuplicates(S a, S b) {
  Pair<LS> pa = parseNGramScorerExpression(a);
  Pair<LS> pb = parseNGramScorerExpression(b);
  ret ai_makeNGramScorer(concatListsToTreeSet(pa.a, pb.a), concatListsToTreeSet(pa.b, pb.b));
}