问题来源：
2个序列时的情形已解决，本文将其扩展到一般情形。
测试代码
#include <cartesian.h>#include <iostream>#include <functional>#include <random>int main() {using type = int;std::vector<type> a = { 10, 9, 2, 1, 1 }, b, c;{for (a1 : a) for (a2 : a) for (a3 : a) for (a4 : a) b.push_back(a1*a2*a3*a4);std::sort(b.begin(), b.end(), std::greater<type>());}{auto it1 = make_cartesian_product(a.begin(), a.end(), a.begin(), a.end(), std::multiplies<type>(), std::less<type>());auto it2 = make_cartesian_product(std::move(it1.first), std::move(it1.second), a.begin(), a.end(), std::multiplies<type>(), std::less<type>());auto it = make_cartesian_product(std::move(it2.first), std::move(it2.second), a.begin(), a.end(), std::multiplies<type>(), std::less<type>());while (it.first != it.second) {c.push_back(*it.first);++it.first;}}std::cout << "equal? " << (b == c) << "\n";for (val : c) std::cout << val << " ";std::cout << "\n";return 0;}
运行结果
，努力爱一个人。付出，不一定会有收获；
