static S ai_matrixGet(S topic, S category) {
  if (has(topic, "is", "category")) ret "yes";
  if (has(topic, "is not", "category")) ret "no";
  try answer joinWithComma(ai_texts_verified(topic, "$X", category));
  S answer = joinWithComma(ai_texts(topic, "$X", category));
  if (nempty(answer)) answer += " [unverified]";
  ret answer;
}