42     std::string s3 = s1; // Mom  in main()  local
43     assert ( std::exchange ( s3, s2 ) == s1 );  in main()
44     assert ( s3 == s2 );  in main()
45     assert ( std::exchange ( s3, "Hi Mom!" ) == s2 );  in main()
46     assert ( s3 == s1 );  in main()
48     s3 = s2; // Dad  in main()
49     assert ( std::exchange ( s3, {} ) == s2 );  in main()
50     assert ( s3.size () == 0 );  in main()
52     s3 = s2; // Dad  in main()
53     assert ( std::exchange ( s3, "" ) == s2 );  in main()
54     assert ( s3.size () == 0 );  in main()