static double combinePercentChanges(Iterable<Double> l) {
  double product = 1;
  for (i : unnull(l))
    if (i != null) product *= (i+100)/100;
  ret product*100-100;
}

static double combinePercentChanges(double... l) {
  double product = 1;
  for (i : unnull(l))
    product *= (i+100)/100;
  ret product*100-100;
}

Began life as a copy of #1019131