static double doubleMultiRatio(Iterable<Double> l) {
  var it = iterator(l);
  if (!it.hasNext()) ret 0;
  double x = it.next();
  while (it.hasNext())
    x = doubleRatio(x, it.next());
  ret x;
}

static double doubleMultiRatio(double... l) {
  int n = l(l);
  if (n == 0) ret 0;
  double x = l[0];
  for (int i = 1; i < n; i++)
    x -= doubleRatio(x, l[i]);
  ret x;
}

Began life as a copy of #1032656