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