Accessing Parent Namespace in C++

I've got a scenario like the following:

class criterion 
{
// stuff about criteria...
};
namespace hex {

     class criterion : public criterion //does not compile
     {                                  //This should inherit from the
     //A hex specific criterion         //criterion class in the global namespace
     };

};

My question is -- how does one inherit from a class in a namspace which is the parent of another namespace?

Billy3