static DoublePt calAverageCenter(CirclesAndLines cal) {
  int n = l(cal.circles);
  if (n == 0) null;
  double x = 0, y = 0;
  for (Circle c : cal.circles) {
    x += c.x; y += c.y;
  }
  ret DoublePt(x/n, y/n);
}