the sum of all fears pretends to be a serious exploration of nuclear terrorism , but 
(ROOT (S (NP (NP (DT the) (NN sum)) (PP (IN of) (NP (DT all) (NNS fears)))) (VP (VP (VBZ pretends) (S (VP (TO to) (VP (VB be) (NP (NP (DT a) (JJ serious) (NN exploration)) (PP (IN of) (NP (JJ nuclear) (NN terrorism)))))))) (, ,) (CC but))))
(ROOT (S (NP (NP (DT ) (NN )) (PP (IN ) (NP (DT ) (NNS )))) (VP (VP (VBZ ) (S (VP (TO ) (VP (VB ) (NP (NP (DT ) (JJ ) (NN )) (PP (IN ) (NP (JJ ) (NN )))))))) (, ) (CC ))))