works on any number of levels 
(ROOT (VP (VBZ works) (PP (IN on) (NP (NP (DT any) (NN number)) (PP (IN of) (NP (NNS levels)))))))
(ROOT (VP (VBZ ) (PP (IN ) (NP (NP (DT ) (NN )) (PP (IN ) (NP (NNS )))))))