, then triple x marks the spot . 
(ROOT (S (, ,) (ADVP (RB then)) (NP (NN triple) (SYM x)) (VP (VBZ marks) (NP (DT the) (NN spot))) (. .)))
(ROOT (S (, ) (ADVP (RB )) (NP (NN ) (SYM )) (VP (VBZ ) (NP (DT ) (NN ))) (. )))