/usr/include/CGAL/Nef_S2/ is in libcgal-dev 4.2-5ubuntu1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/CGAL/Nef_S2/Generic_handle_map.h
/usr/include/CGAL/Nef_S2/Normalizing.h
/usr/include/CGAL/Nef_S2/OGL_base_object.h
/usr/include/CGAL/Nef_S2/SM_checker.h
/usr/include/CGAL/Nef_S2/SM_const_decorator.h
/usr/include/CGAL/Nef_S2/SM_constrained_triang_traits.h
/usr/include/CGAL/Nef_S2/SM_decorator.h
/usr/include/CGAL/Nef_S2/SM_decorator_traits.h
/usr/include/CGAL/Nef_S2/SM_io_parser.h
/usr/include/CGAL/Nef_S2/SM_items.h
/usr/include/CGAL/Nef_S2/SM_iteration.h
/usr/include/CGAL/Nef_S2/SM_list.h
/usr/include/CGAL/Nef_S2/SM_overlayer.h
/usr/include/CGAL/Nef_S2/SM_point_locator.h
/usr/include/CGAL/Nef_S2/SM_triangulator.h
/usr/include/CGAL/Nef_S2/SM_visualizor.h
/usr/include/CGAL/Nef_S2/Sphere_circle.h
/usr/include/CGAL/Nef_S2/Sphere_direction.h
/usr/include/CGAL/Nef_S2/Sphere_geometry.h
/usr/include/CGAL/Nef_S2/Sphere_geometry_OGL.h
/usr/include/CGAL/Nef_S2/Sphere_map.h
/usr/include/CGAL/Nef_S2/Sphere_point.h
/usr/include/CGAL/Nef_S2/Sphere_segment.h
/usr/include/CGAL/Nef_S2/Sphere_triangle.h
/usr/include/CGAL/Nef_S2/leda_sphere_map.h
/usr/include/CGAL/Nef_S2/sphere_predicates.h