 Hold on a second.
( ( Hold ( on ( a second ) ) ) . )
(ROOT (S (VP (VB Hold) (PP (IN on) (NP (DT a) (NN second)))) (. .)))
Wait a second.
( ( Wait ( a second ) ) . )
(ROOT (S (VP (VB Wait) (NP (DT a) (NN second))) (. .)))