of "forwarding constructors".
Quoting some comment in the code:
"
This is a simple tag which is used as additional (first) argument in
some kernel functors, to tell them to return the base (rep) class,
instead of the main type (e.g. Kernel_base::Point_2 instead
of Kernel::Point_2). This is a minor optimization which prevents
useless copies of the "reps".
Those functors are only those used in the constructors of the kernel
types like Point_2, so it's limited.
The real solution will be to use "forwarding constructors", when they
will be available in C++.
In the mean time, this should be a mostly/hopefully internal hack.
"