from X cons X from s X sel 0 cons X XS X sel s N cons X XS sel N XS minus X 0 0 minus s X s Y minus X Y quot 0 s Y 0 quot s X s Y s quot minus X Y s Y zWquot XS nil nil zWquot nil XS nil zWquot cons X XS cons Y YS cons quot X Y zWquot XS YS 2.2 quot s X s Y minus X Y sel s N cons X XS sel N XS from X from s X minus s X s Y minus X Y zWquot cons X XS cons Y YS quot X Y quot s X s Y quot minus X Y s Y zWquot cons X XS cons Y YS zWquot XS YS true zWquot cons X XS cons Y YS zWquot XS YS true s 1 0 zWquot 2 2 0 minus 2 0 zWquot 2 0 0 0 0 quot 2 0 sel 2 0 from 1 0 sel 2 0 nil 0 0 minus 2 0 from 1 0 cons 2 2 1 quot 2 0 zWquot cons X XS cons Y YS quot X Y false quot s X s Y quot minus X Y s Y true s 1 2 zWquot 2 0 minus 2 1 zWquot 2 0 0 0 1 quot 2 0 sel 2 0 from 1 0 sel 2 0 nil 0 0 minus 2 0 from 1 0 cons 2 1 quot 2 1 0 minus X 0 0 minus s X s Y minus X Y sel s N cons X XS sel N XS true s 1 1 1 zWquot 2 0 minus 2 1 zWquot 2 0 0 0 1 quot 2 0 sel 2 1 2 0 from 1 0 sel 2 0 nil 0 0 minus 2 0 from 1 0 cons 2 2 11293 quot 2 1 0 minus X 0 0 minus s X s Y minus X Y from X from s X true Failed! NaTT certifiable-1.6