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