CGAL 5.4.4  2D and 3D Linear Geometry Kernel

AdaptableFunctor
(with one argument)CGAL::Triangle_2<Kernel>
Operations  
Kernel::Triangle_2  operator() (const Kernel::Triangle_2 &t) 
returns the triangle with opposite orientation to t (this flips the positive and the negative side, but not bounded and unbounded side).  