// angle between p1 and p2 with respect to center point
// result is between 0 and pi()*2
static double threePointAngle(IDoublePt p1, IDoublePt center, IDoublePt p2) {
  double angle = atan2(
      p1.x_double()-center.x_double(),
      p1.y_double()-center.y_double())
    -atan2(
      p2.x_double()-center.x_double(),
      p2.y_double()-center.y_double());
  ret mod(angle, pi()*2);
}