every once in a while a film 
(ROOT (FRAG (ADVP (DT every) (RB once) (PP (IN in) (NP (DT a) (NN while)))) (NP (DT a) (NN film))))
(ROOT (FRAG (ADVP (DT ) (RB ) (PP (IN ) (NP (DT ) (NN )))) (NP (DT ) (NN ))))