// properties:
// -yields a proper probability (0 < p <= 1)
// -distance 0 has probability 1
// -function is monotonically decreasing
//    (greater distance = lower probability)
// -negative distances are interpreted as positive
static double genericDistanceToProbability(double distance) {
  ret 1/(1+abs(distance));
}

static double genericDistanceToProbability(double a, double b) {
  ret genericDistanceToProbability(a-b);
}