inline const Angle operator / ( const Angle & a1, const Angle & a2 ) { return Angle( a1.radian() / a2.radian() ); }
inline const Angle operator * ( const Angle & angle, const double radian ) { return Angle( angle.radian() * radian ); }
inline const Angle operator - ( const Angle & angle ) { return Angle( -angle.radian() ); }