class MyTrajectoryBfield : public MyTrajectory {
 public:
  MyTrajectoryBfield(const DMagneticFieldMap *bfield, int level = 1);
  void swim(const HepVector &param);
  void swim(double startingXprime, double startingZ, double startingTheta, double startingPhi, double pinv);
  unsigned int getNumberOfParams();
  virtual vector<double> getDelta() {
    return delta;
  }
 private:
  HepVector B;
  const DMagneticFieldMap *bfield;
  unsigned int nparams;
  vector<double> delta;
  int debug_level;
};