The details of what happened on the morning of September 11 are complex, but they play out a simple theme.
( ( ( ( ( ( ( The details ) ( of ( what ( happened ( on ( ( the morning ) ( of ( September 11 ) ) ) ) ) ) ) ) ( are complex ) ) , ) but ) ( they ( ( play out ) ( a ( simple theme ) ) ) ) ) . )
(ROOT (S (S (NP (NP (DT The) (NNS details)) (PP (IN of) (SBAR (WHNP (WP what)) (S (VP (VBD happened) (PP (IN on) (NP (NP (DT the) (NN morning)) (PP (IN of) (NP (NNP September) (CD 11)))))))))) (VP (VBP are) (ADJP (JJ complex)))) (, ,) (CC but) (S (NP (PRP they)) (VP (VBP play) (PRT (RP out)) (NP (DT a) (JJ simple) (NN theme)))) (. .)))
The theme of the events on September 11 is simple even though the details are complex. 
( ( ( The theme ) ( of ( ( the events ) ( on ( September 11 ) ) ) ) ) ( ( ( is simple ) ( even ( though ( ( the details ) ( are complex ) ) ) ) ) . ) )
(ROOT (S (NP (NP (DT The) (NN theme)) (PP (IN of) (NP (NP (DT the) (NNS events)) (PP (IN on) (NP (NNP September) (CD 11)))))) (VP (VBZ is) (ADJP (JJ simple)) (SBAR (RB even) (IN though) (S (NP (DT the) (NNS details)) (VP (VBP are) (ADJP (JJ complex)))))) (. .)))