CGAL 5.6.1  2D and 3D Linear Geometry Kernel

AdaptableBinaryFunction
CGAL::compare_slope()
Operations  
Comparison_result  operator() (const Kernel::Point_3 &p, const Kernel::Point_3 &q, const Kernel::Point_3 &r, const Kernel::Point_3 &s) 
compares the slopes of the segments (p,q) and (r,s) , where the slope is the variation of the z coordinate from the first to the second point of the segment divided by the length of the segment.  