CGAL 5.6.1  2D and 3D Linear Geometry Kernel

AdaptableBinaryFunction
CGAL::compare_xy()
Operations  
bool  operator() (const Kernel::Point_3 &p, const Kernel::Point_3 &q) 
returns true iff p and q have the same Cartesian \( x\)coordinate and the same Cartesian \( y\)coordinate.  