//  Point-Extent pattern for binary arithmetic operators


 PointClass& operator+=(const Class rs);
 PointClass& operator-=(const Class rs);
 
 PointClass  operator+ (const Class rs) const {return PointClass(*this)+=rs;}
 PointClass  operator- (const Class rs) const {return PointClass(*this)-=rs;}
 Class       operator- (const PointClass rs) const {return Class(value-rs.value);}

friend
 PointClass  operator+ (const Class ls, const PointClass rs)
    {return PointClass(rs)+=ls;}