/in/foo.cc:2:17: error: 'tsd' is not a namespace-name
 using namespace tsd;
                 ^~~
/in/foo.cc:2:20: error: expected namespace-name before ';' token
 using namespace tsd;
                    ^
/in/foo.cc: In function 'int main()':
/in/foo.cc:5:3: error: 'cout' was not declared in this scope
   cout<<"23";
   ^~~~
/in/foo.cc:5:3: note: suggested alternative:
In file included from /in/foo.cc:1:0:
/usr/include/c++/6/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~