static double digitalVariation(int[] array) {
  int n = l(array);
  if (n == 0) ret 0; // ? - undefined
  if (n == 1) ret 0;
  var value = array[0];
  int deviations = 0;
  for (int i = 1; i < n; i++)
    if (array[i] != value)
      ++deviations;
  ret doubleRatio(deviations, n);
}

static <A> double digitalVariation(Iterable<A> l) {
  var it = iterator(l);
  if (!it.hasNext()) ret 0; // ? - undefined
  int n = 1;
  var value = it.next();
  int deviations = 0;
  while (it.hasNext()) {
    ++n;
    if (!eq(value, it.hasNext()))
      ++deviations;
  }
  ret doubleRatio(deviations, n);
}