of a masterpiece -- and a challenging one 
(ROOT (PP (IN of) (NP (NP (DT a) (NN masterpiece)) (, --) (CC and) (NP (DT a) (JJ challenging) (NN one)))))
(ROOT (PP (IN ) (NP (NP (DT ) (NN )) (, ) (CC ) (NP (DT ) (JJ ) (NN )))))