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 = dropFirstWord(s);
    S v = firstVerb(s);
    if (v != null) ret a + " " + v + "?";
  }
  if (isVerb(a)) ret a + "?";
  null;
}