#include <string>
#include <iostream>
#include <list>

using namespace std;

typedef list<string> LIST_STR;

int main()
{

    list<LIST_STR> ls;

    ls.emplace_back();
    ls.emplace_back();
    ls.emplace_back();

    for (auto &x: ls) {
        x.push_back("Hello ");
        x.push_back("world");
        x.push_back("!\n");
    }

    for (auto &x: ls) {
        for (auto &xs: x) {
            cout << xs;
        }
    }

    return 0;
}