function showSum(shows) {
  var result = function (sum) {
    return sum.fold(function (idx, n, value) {
      return "Sum(" + idx + "/" + n + ": " + shows[idx](value) + ")";
    });
  };

  return utils.curried2(result, arguments);
}
- `show.sum(shrinks: (a -> string, b -> string...), x: (a | b ...)): string`