Err...I don't know.
( Err ( ... ( ( I ( ( do n't ) know ) ) . ) ) )
(ROOT (FRAG (NP (NNP Err)) (: ...) (S (NP (PRP I)) (VP (VBP do) (RB n't) (VP (VB know)))) (. .)))
I know.
( I ( know . ) )
(ROOT (S (NP (PRP I)) (VP (VBP know)) (. .)))