protected vs private (Simplex_tree)

[mglisse]

In the Simplex_tree, many types, functions and members are marked private. I wonder if we should mark some of them protected instead. The effect would be the same for someone using the Simplex_tree directly, but it would give advanced users (and possibly ourselves?) a chance to implement additional functions in a derived class, which may be quite hard using only the public interface.
That doesn't mean that undocumented private details would become stable interfaces though, anyone using them would still have to check with each release if things still work.