// angle is in radians for faster calculations
srecord PolarPt(double length, double angle) {
  DoublePt toDoublePt() {
    ret doublePt(length*cos(angle), length*sin(angle));
  }
  
  meta-for Pt also as DoublePt {
    *(Pt p) {
      angle = atan2(p.x, p.y);
      length = p.length();
    }
  }
}