static double doubleSum(Iterable<Double> l) {
  double sum = 0;
  for (Double i : unnull(l))
    if (i != null) sum += i;
  ret sum;
}

static double doubleSum(double... l) {
  double sum = 0;
  if (l != null) for (double i : l)
    sum += i;
  ret sum;
}

static <A> double doubleSum(Iterable<A> l, IF1_ObjectToDouble<A> f) {
  double sum = 0;
  for (a : l) sum += f.get(a);
  ret sum;
}

static <A> double doubleSum(Iterable<A> l, S field) {
  double sum = 0;
  for (a : l) sum += toDouble(getOpt(a, field));
  ret sum;
}