int main() {

    String s1 = "aaa";
    String s2("Hello");
    String s3(s2);
    String s4 = s1;
    String s5("a");
    String s6;

    s1 += s2;
    s2 = s3;
    s3 += s4;
    s4 = "NEU";


    cout << "1. expected: 'aaaHello' -- output : " << s1 << endl;
    cout << "2. expected: 'l' \t\t-- output : "   << s2[ 2 ] << endl;
    cout << "3. expected: 'Hello' \t-- output : "   << s2 << endl;
    cout << "4. expected: 'Helloaaa' -- output : "   << s3 << endl;
    cout << "5. expected: 'NEU' \t\t-- output : "   << s4 << endl;
    cout << "6. expected: 'a' \t\t-- output : "   << s5 << endl;
    cout << "7. expected: '' \t\t-- output : "   << s6 << endl;



}