the surfers off this lisp were back refer .
(ROOT_0 (S_0 (ATS_3 (NP_1 (NP_6 (DT_0 the) (NNS_1 surfers)) (PP_0 (IN_1 off) (NP_5 (DT_0 this) (NN_2 lisp)))) (VP_4 (ATVP_1 (VBD_0 were) (RB_1 back)) (VP_5 (VBP_0 refer)))) (DOT_0 .)))