code: function Ellipse(rx, ry, ax) {
    if (!(this instanceof Ellipse)) { return new Ellipse(rx, ry, ax); }
    this.rx = rx;
    this.ry = ry;
    this.ax = ax;
  }
docs: Class constructor : an ellipse centred at 0 with radii rx,ry and x - axis - angle ax.