code: def _gap_progenitor_setup(self):
          
          self._gap_progenitor= self._progenitor().flip() # new orbit, flip velocities
          # Make sure we do not use physical coordinates
          self._gap_progenitor.turn_physical_off()
          # Now integrate backward in time until tdisrupt
          ts= numpy.linspace(0.,self._tdisrupt,1001)
          self._gap_progenitor.integrate(ts,self._pot)
          # Flip its velocities, should really write a function for this
          self._gap_progenitor._orb.orbit[:,1]= -self._gap_progenitor._orb.orbit[:,1]
          self._gap_progenitor._orb.orbit[:,2]= -self._gap_progenitor._orb.orbit[:,2]
          self._gap_progenitor._orb.orbit[:,4]= -self._gap_progenitor._orb.orbit[:,4]
          return None
docs: Setup an Orbit instance that's the progenitor integrated backwards