Exclusive charmless $B_s$ hadronic decays into $\eta'$ and $\eta$
  Using the next-to-leading order QCD-corrected effective Hamiltonian, charmless exclusive nonleptonic decays of the $B_s$ meson into $\eta$ or $\eta'$ are calculated within the generalized factorization approach. Nonfactorizable contributions are included with two different treatments. Some subtleties involved are discussed. 
B. Tseng