prefer to not use `using namespace`

[ASmoliak]

In source files, prefer to use a fully qualified namespace, rather than using namespace
In header files, you should NEVER write using namespace std or any other namespace, the reason for this is that it will propagate to any other header or source file that includes it, and will end up causing a lot of namespace pollution, compilation issues, and possibly even runtime errors if you're not lucky.

[yonicswe]

sure. done.