### Commits that were reverted and might be useful to restore in CGAL-6.1...
- [ ] the fix about the CMake warning on VTK and `VTK_USE_FILE` f39b90670b5812801c86abfe818de3a18d2c134b, because it seems we still test with a VTK version 8.2,
- [ ] and the experiment to make `Segment_3` tuple-like ac47f30cd8a0921e52e3bd2a88fb435092eef837, because it is useless, and make Apple Clang emit a dubious warning, that is not easy to work around.
- [ ] use `boost::unordered_flat_map` to optimize `Polyline_constraint_hierarchy_2`: 1ca6c17a2a165dccb2204f01d38d6d3d903d11c9.
### Functor to document in the `Kernel` concept
- [ ] add `Compare_angle_3(Point_3, Point_3, Point_3, Point_3, Point_3, Point_3)` (with six points) **TODO: needs doc**
Originally posted by @lrineau in https://github.com/CGAL/cgal/issues/8170#issuecomment-2124715682: