CGAL 6.0.1  2D and 3D Linear Geometry Kernel

AdaptableBinaryFunction
CGAL::compare_xy()
Operations  
A model of this concept must provide:  
bool  operator() (const Kernel::Point_3 &p, const Kernel::Point_3 &q) 
returns true iff the \( x\)coordinate of p is smaller than the \( x\)coordinate of q or if they are the same and the \( y\)coordinate of p is smaller than the \( y\)coordinate of q .  