9 std::ostream& operator<<( std::ostream& os_, const N2::C0& m_);
13 friend std::ostream& operator<<(std::ostream& os, const C1& what);
17 friend std::ostream& operator<<(std::ostream& os, const C2& what);
20 void foo(std::ostream & os, const N2::C0& m)
22 os << m; // Is this line valid?