// points are oriented clockwise, starting at top
static L<Pt> pointsAroundCircle(Pt center, int n, double radius) {
  L<Pt> l = emptyList(n);
  double step = doubleRatio(pi()*2, n), angle = 0;
  for i to n: {
    l.add(pointOnCircle(center, radius, angle));
    angle += step;
  }
  ret l;
}