def subset_sum(values, target):
    if type(target) is not int or target < 0 or any(type(value) is not int or value < 0 for value in values):
        raise ValueError("Nonnegative integer inputs required")

    def choose(index, remaining):
        if remaining == 0:
            return []
        if index == len(values) or remaining < 0:
            return None
        included = choose(index + 1, remaining - values[index])
        if included is not None:
            return [index] + included
        return choose(index + 1, remaining)

    return choose(0, target)

values = [7, 4, 6]
chosen = subset_sum(values, 11)
print(chosen)
print(sum(values[index] for index in chosen))
print(subset_sum([4, 6], 3))