( a ) strong piece of work 
(ROOT (FRAG (-LRB- -LRB-) (LS a) (-RRB- -RRB-) (NP (NP (JJ strong) (NN piece)) (PP (IN of) (NP (NN work))))))
(ROOT (FRAG (-LRB- ) (LS ) (-RRB- ) (NP (NP (JJ ) (NN )) (PP (IN ) (NP (NN ))))))