to think of any film more challenging or depressing than the grey zone 
(ROOT (S (VP (TO to) (VP (VB think) (PP (IN of) (NP (NP (DT any) (NN film)) (ADJP (ADJP (RBR more) (JJ challenging) (CC or) (JJ depressing)) (PP (IN than) (NP (DT the) (JJ grey) (NN zone))))))))))
(ROOT (S (VP (TO ) (VP (VB ) (PP (IN ) (NP (NP (DT ) (NN )) (ADJP (ADJP (RBR ) (JJ ) (CC ) (JJ )) (PP (IN ) (NP (DT ) (JJ ) (NN ))))))))))