code: def _find_closest_trackpoint(self,R,vR,vT,z,vz,phi,interp=True,xy=False,
                                   usev=False):
          
          return self.find_closest_trackpoint(R,vR,vT,z,vz,phi,
                                              interp=interp,xy=xy,
                                              usev=usev)
docs: For backward compatibility