Who? asked Tommy.
( ( ( Who ? ) ( asked Tommy ) ) . )
(ROOT (S (S (FRAG (WHNP (WP Who)) (. ?)) (NP (JJ asked)) (VP (VBZ Tommy))) (. .)))
Tommy didn't know, who.
( ( ( ( Tommy ( ( did n't ) know ) ) , ) who ) . )
(ROOT (S (S (NP (NNP Tommy)) (VP (VBD did) (RB n't) (VP (VB know)))) (, ,) (FRAG (WHNP (WP who))) (. .)))