#include <iostream> 
#include <vector> 
 
int main(){
 
    std::vector<std::vector<int>> vec = {{0,1},{2,3}};
 
    vec.push_back({4,5});
    vec.push_back({5,6});
 
    for (int i = 0; i < vec.size(); i++){
        for (int i2 = 0; i2 < vec[i].size(); i2++){
            std::cout << vec[i][i2] << std::endl; 
        }
		/* alternatively:
        auto &vec2 = vec[i];
        for (size_t i2 = 0; i2 < vec2.size(); ++i2){
            std::cout << vec2[i2] << std::endl; 
        }
        */
	}
 
    return 0;
}

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