  We review Koszul duality in representation theory of category $ \cal O $,
especially we give a new presentation of the Koszul duality functor. Combining
this with work of Backelin, we show that the translation and Zuckerman functors
are Koszul dual to each other, thus verifying a conjecture of Bernstein,
Frenkel and Khovanov. Finally we use Koszul duality to give a short proof of
the Enright-Shelton equivalence.
