// 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:
    l.add(pointOnCircle(center, radius, i*factor));
  ret l;
}