text: class UnaryFactor: public NoiseModelFactor1<Pose2> {
    double mx_, my_; ///< X and Y measurements
  
  public:
    UnaryFactor(Key j, double x, double y, const SharedNoiseModel& model):
      NoiseModelFactor1<Pose2>(model, j), mx_(x), my_(y) {}
  
    Vector evaluateError(const Pose2& q,
                         boost::optional<Matrix&> H = boost::none) const
    {
      const Rot2& R = q.rotation();
      if (H) (*H) = (gtsam::Matrix(2, 3) <<
              R.c(), -R.s(), 0.0,
              R.s(), R.c(), 0.0).finished();
      return (Vector(2) << q.x() - mx_, q.y() - my_).finished();
    }
  };
  