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