static LineThroughPoint perpendicularBisector(IDoublePt a, IDoublePt b) {
  DoublePt midpoint = doublePt((a.x_double()+b.x_double())/2, (a.y_double()+b.y_double())/2);
  double angle = atan2(b.y_double()-a.y_double(), a.x_double()-b.x_double());
  ret LineThroughPoint(midpoint, angle);
}