purgenilnilpurge.xy.xpurgeremovexyremovexnilnilremovex.yzif=xyremovexz.yremovexzpurge.xypurgeremovexypurge.xyremovexyremovex.yzremovexzremovex.yzremovexztruepurge.xypurgeremovexytrue121purge111.21remove20purge11nil00if30=20purge.xyremovexyfalse1removex.yzremovexztrue1remove212.2112purge111nil00remove212if31=20