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