// converts the index of a correct guess in a probabilistic
// list to a score between 0 and 1

// properties:
// -index 0 gets score 1
// -higher indices get lower scores
// -index -1 (not found) gets score 0
static double scoreForIndexInProbabilisticList(int index) {
  ret index < 0 ? 0 : distanceToProbability(index);
}

Began life as a copy of #1035312