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