I made a test for circulating through output points of alpha_shape_2. There is a square at vertices (0 0), (10 0), (10 10), (0 10).
Obviously, `A.alpha_shape_vertices_begin()` begins with (10 0) and the incident vertex chosen by `incident_vertices` was (10, 10). Circulation was going good until I got a weird output (0.5 0.5) which should have been (10 0).
I thought that (0.5 0.5) is past-end-value although circulators have no such thing. Thats why I tried `--vccopy` to stop circulation earlier and avoid (0.5 0.5). This cheap trick works for this specific case but not in general.
I also checked edges. The same weird number (0.5 0.5) appears again.
My ultimate goal is to make a simple polygon from points returned by alpha_shape_2. Purpose of circulating is to get ordered points.