teetering on the edge of sanity 
(ROOT (S (VP (VBG teetering) (PP (IN on) (NP (NP (DT the) (NN edge)) (PP (IN of) (NP (NN sanity))))))))
(ROOT (S (VP (VBG ) (PP (IN ) (NP (NP (DT ) (NN )) (PP (IN ) (NP (NN ))))))))