piecesOf function
The rings of f's band cut at merged (spec 08 D9), in u order, start
piece first: each simple and anticlockwise in the host's local space.
merged must not be empty.
Implementation
List<List<Vector2>> piecesOf(HostFrame f, List<(double, double)> merged) =>
[for (final p in _pieces(f, merged)) p.ring];