// points are oriented clockwise, starting at top
static L<Pt> pointsAroundCircle(Pt center, int n, double radius) {
  L<Pt> l = emptyList(n);
  double factor = doubleRatio(pi()*2, n);
  for i to n: {
    double angle = i*factor;
    l.add(pt(
      iround(center.x + sin(angle)*radius),
      iround(center.y - cos(angle)*radius)));
  }
  ret l;
}