's the mark of a documentary that works 
(ROOT (VP (VBZ 's) (NP (NP (DT the) (NN mark)) (PP (IN of) (NP (NP (DT a) (NN documentary)) (SBAR (WHNP (WDT that)) (S (VP (VBZ works)))))))))
(ROOT (VP (VBZ ) (NP (NP (DT ) (NN )) (PP (IN ) (NP (NP (DT ) (NN )) (SBAR (WHNP (WDT )) (S (VP (VBZ )))))))))