code: def FinalSpin( Xi, eta ):
      
      s4 = -0.129
      s5 = -0.384
      t0 = -2.686
      t2 = -3.454
      t3 = 2.353
      etaXi = eta * Xi
      eta2 = eta*eta
      finspin = (Xi + s4*Xi*etaXi + s5*etaXi*eta + t0*etaXi + 2.*(3.**0.5)*eta + t2*eta2 + t3*eta2*eta)
      if finspin > 1.0:
          raise ValueError("Value of final spin > 1.0. Aborting")
      else:
          return finspin
docs: Computes the spin of the final BH that gets formed after merger. This is done usingn Eq 5-6 of arXiv:0710.3345