it is impossible to think of any film more challenging or depressing than the grey zone 
(ROOT (S (NP (NP (PRP it))) (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 (S (NP (NP (PRP ))) (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 ))))))))))))