static void ai_spec_executeBestSplit(S s) {
  if (!isNormalQuoted(s)) ret;
  int idx = ai_intOrMinus1(s, "is best split at index", "$X");
  if (idx < 0) ret;
  post(s, "best-splits to", sfu(splitStringAtIndices(unquote(s), ll(idx))));
}