foo.cc:2:11: error: expected identifier before 'using'
    2 | namespace using std;
      |           ^~~~~
foo.cc:2:17: error: expected nested-name-specifier before 'std'
    2 | namespace using std;
      |                 ^~~
foo.cc: In function 'int main()':
foo.cc:4:2: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    4 |  cout<<"0";
      |  ^~~~
      |  std::cout
In file included from foo.cc:1:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~

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