class MyTrajectoryBfield : public MyTrajectory {
 public:
  MyTrajectoryBfield(const DMagneticFieldMap *bfield);
  void swim(const HepVector &param);
  void swim(double charge, HepVector startingPoint, double p, double theta, double phi);
  void swim(double startingX, double startingY, double startingTheta, double startingPhi, double curvature);
  unsigned int getNumberOfParams();
  virtual vector<double> getDelta() {
    return delta;
  }
 private:
  HepVector B;
  const DMagneticFieldMap *bfield;
  unsigned int nparams;
  vector<double> delta;
};