docs: def trajectory_SgConst(Sg=0.1, delta_logt_dex=-0.01):
      '''
      setup trajectories for constant radiation entropy.
  
      S_gamma/R where the radiation constant R = N_A*k
      (Dave Arnett, Supernova book, p. 212)
      This relates rho and T but the time scale for this
      is independent.
  
      Parameters
      ----------
      Sg : float
          S_gamma/R, values between 0.1 and 10. reflect conditions in
          massive stars.  The default is 0.1.
      delta_logt_dex : float
          Sets interval between time steps in dex of logtimerev.  The
          default is -0.01.
  
      '''
  
      # reverse logarithmic time
      logtimerev=np.arange(5.,-6.,delta_logt_dex)
      logrho=np.linspace(0,8.5,len(logtimerev))
      logT = (old_div(1.,3.))*(logrho + 21.9161 + np.log10(Sg))
  
      #rho_6=10**logrho/(0.1213*1.e6)
      #T9=rho_6**(1./3.)
      #logT_T3=np.log10(T9*1.e9)
  
  
      pl.close(3);pl.figure(3);pl.plot(logrho,logT,label='$S/\mathrm{N_Ak}='+str(Sg)+'$')
      pl.legend(loc=2);pl.xlabel('$\log \\rho$'); pl.ylabel('$\log T$')
      pl.close(5);pl.figure(5);pl.plot(logtimerev, logrho)
      pl.xlabel('$\log (t_\mathrm{final}-t)$'); pl.ylabel('$\log \\rho$')
      pl.xlim(8,-6)
  
      pl.close(6);pl.figure(6);pl.plot(logtimerev)
      pl.ylabel('$\log (t_\mathrm{final}-t)$'); pl.xlabel('cycle')
  
      # [t] logtimerev yrs
      # [rho] cgs
      # [T]   K
  
      T9=old_div(10**logT,1.e9)
      data=[logtimerev,T9,logrho]
      att.writeTraj(filename='trajectory.input', data=data, ageunit=2, tunit=1, rhounit=1, idNum=1)