// find best threshold value to separate list1 (smaller values) from list2 (bigger values), minimizing false positives + false negatives
// returns (threshold, false positives, false negatives)
static T3<Double, Int, Int> findBestThreshold(Cl<Double> list1, Cl<Double> list2) {
  if (empty(list1) || empty(list2)) null;
  L<Double> l1 = sorted(list1);
  L<Double> l2 = sorted(list2);
  double threshold = avg(doubleMax(l1), doubleMin(l2));
  int falsePositives = countPred(l1, d -> d > threshold); // can be optimized
  int falseNegatives = countPred(l2, d -> d <= threshold); // ditto
  if (falsePositives > falseNegatives) {
  } else if (falseNegatives > falsePositives) {
  }
  ret t3(threshold, falsePositives, falseNegatives);
}