Provide an easy way to have K::Point (shorter than K::Type<Point_tag>::type...). Could do the same for functors, maybe.
Now that I have an Object_list, it would be possible to use directly K::Point and get rid of that Type<Point_tag>::type horror, which sure looks simpler (although it is actually more complicated behind), but it has some drawbacks. In particular, in a CRTP context, typedef std::pair<Derived::Point> Segment; won't work (it worked inside Type<...>), one has to use typedef MySegment<Derived> Segment; instead (C++ can be a pain...).