But fortunately they didn't.
( But ( fortunately ( they ( ( did n't ) . ) ) ) )
(ROOT (S (CC But) (ADVP (RB fortunately)) (NP (PRP they)) (VP (VBD did) (RB n't)) (. .)))
Fortunately, they did not do that.
( Fortunately ( , ( they ( ( ( did not ) ( do that ) ) . ) ) ) )
(ROOT (S (ADVP (RB Fortunately)) (, ,) (NP (PRP they)) (VP (VBD did) (RB not) (VP (VB do) (NP (DT that)))) (. .)))