static S ai_guessVerb(S s) {
  S a = firstWord(s);
  if (a == null) null;
  if (ai_questionWord_1(a)) { s = dropFirstWord(s); a = firstWord(s); }
  if (ai_yesNoQuestionWord_1(a) && neqicOneOf(a, "am", "are", "is")) {
    S s2 = dropFirstWord(s);
    S v = firstVerb(s2);
    if (v != null)
      if (eqic(a, "does")) ret verbToThirdPerson(v) + "?";
      else if (eqic(a, "do")) ret v + "?";
      else if (eqic(a, "did")) ret verbToPast(v) + "?";
      else ret a + " " + v + "?";
  }
  if (isVerb(a)) ret a + "?";
  s = ai_skipNounPhrase(s);
  ret firstVerb(s);
}