a copy of a copy of a copy 
(ROOT (NP (NP (DT a) (NN copy)) (PP (IN of) (NP (NP (DT a) (NN copy)) (PP (IN of) (NP (DT a) (NN copy)))))))
(ROOT (NP (NP (DT ) (NN )) (PP (IN ) (NP (NP (DT ) (NN )) (PP (IN ) (NP (DT ) (NN )))))))