static S ai_matrix_verbToAnswer(S s) {
  if (eqic(s, "is")) ret "yes";
  if (eqic(s, "is not")) ret "no";
  new Matches m;
  if (ewic(s, " is", m))
    ret m.rest();
  if (ewic(s, " is not", m))
    ret m.rest() + " not";
  null;
}