a popcorn film , not a must-own , 
(ROOT (NP (NP (DT a) (NN popcorn) (NN film)) (, ,) (RB not) (NP (DT a) (S (VP (NN must) (HYPH -) (VP (JJ own))))) (, ,)))
(ROOT (NP (NP (DT ) (NN ) (NN )) (, ) (RB ) (NP (DT ) (S (VP (NN ) (HYPH ) (VP (JJ ))))) (, )))