It isn't, of course.
( It ( ( ( ( is n't ) , ) ( of course ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is) (RB n't)) (, ,) (X (PP (IN of) (NP (NN course)))) (. .)))
It is, of course.
( It ( ( ( is , ) ( of course ) ) . ) )
(ROOT (S (NP (PRP It)) (VP (VBZ is)) (, ,) (X (PP (IN of) (NP (NN course)))) (. .)))