Prove how sort was insufficient

sort.cc

#include <algorithm>
#include <iostream>
#include <vector>

static bool cmp(int a, int b) {
  return ((a == 3 && b == 0)
          || (a == 4 && b == 2)
          || (a == 5 && b == 1));
}

template <typename V>
void print(const V& ary) {
  for (auto a: ary) std::cout << " " << a;
}

int main() {
  std::vector<int> ary = {0, 1, 2, 3, 4, 5};
  do {
    std::vector<int> sorted = ary;
    std::stable_sort(sorted.begin(), sorted.end(), cmp);
    for (int i = 0; i < 6 - 1; ++i)
      for (int j = i + 1; j < 6; ++j)
        if (cmp(sorted[j], sorted[i])) {
          print(ary);
          std::cout << " =>";
          print(sorted);
          std::cout << " (" << sorted[i] << "," << sorted[j] << ")" << std::endl;
          goto quit;
        }
#if 0
    print(sorted);
    std::cout << std::endl;
#endif
  quit:;
  } while (std::next_permutation(ary.begin(), ary.end()));
  return 0;
}