They lower and higher refer that to stop .
(ROOT_0 (S_0 (ATS_3 (ATS_1 (NP_1 (PRP_0 They)) (ADVP_0 (ATADVP_4 (JJR_0 lower) (CC_0 and)) (JJR_0 higher))) (VP_3 (VBP_0 refer) (SBAR_0 (WHNP_0 (WDT_0 that)) (S_2 (VP_0 (TO_0 to) (VP_1 (VB_0 stop))))))) (DOT_0 .)))