Look, there's a legend here.
( Look ( , ( there ( ( ( 's ( a legend ) ) here ) . ) ) ) )
(ROOT (S (S (VP (VB Look))) (, ,) (NP (EX there)) (VP (VBZ 's) (NP (DT a) (NN legend)) (ADVP (RB here))) (. .)))
There isn't a legend here.
( There ( ( ( ( is n't ) ( a legend ) ) here ) . ) )
(ROOT (S (NP (EX There)) (VP (VBZ is) (RB n't) (NP (DT a) (NN legend)) (ADVP (RB here))) (. .)))