Skyline (Direction sky);
Skyline (vector<Box> const &bldgs, Axis a, Direction sky);
Skyline (vector<Drul_array<Offset> > const &bldgs, Axis a, Direction sky);
Skyline (Direction sky);
Skyline (vector<Box> const &bldgs, Axis a, Direction sky);
Skyline (vector<Drul_array<Offset> > const &bldgs, Axis a, Direction sky);