code: def _setup_progIsTrack(self):
          
          # We need to flip the sign of the offset, to go to the progenitor
          self._sigMeanSign*= -1.
          # Use _determine_stream_track_single to calculate the track-progenitor
          # offset at zero angle separation
          prog_stream_offset=\
              _determine_stream_track_single(self._aA,
                                             self._progenitor,
                                             0., #time = 0
                                             self._progenitor_angle,
                                             self._sigMeanSign,
                                             self._dsigomeanProgDirection,
                                             lambda x: self.meanOmega(x,use_physical=False),
                                             0.) #angle = 0
          # Setup the new progenitor orbit
          progenitor= Orbit(prog_stream_offset[3])
          # Flip the offset sign again
          self._sigMeanSign*= -1.
          # Now re-do the previous setup
          self._progenitor_setup(progenitor,self._leading,False)
          self._offset_setup(self._sigangle,self._leading,
                             self._deltaAngleTrack)
          return None
docs: If progIsTrack, the progenitor orbit that was passed to the 
          streamdf initialization is the track at zero angle separation;
          this routine computes an actual progenitor position that gives
          the desired track given the parameters of the streamdf