#feature on safety
#include <std2.h>

int main() safe {
  std2::vector<int> vec { 11, 15, 20 };

  for(int x : vec) {
    // Ill-formed. mutate of vec invalidates iterator in ranged-for.
    if(x % 2)
      mut vec.push_back(x);

    std2::println(x);
  }
}