zeros cons 0 n__zeros U11 tt L U12 tt activate L U12 tt L s length activate L length nil 0 length cons N L U11 tt activate L zeros n__zeros activate n__zeros zeros activate X X 2.2 U11 tt L activate L U12 tt L length activate L U12 tt L activate L length cons N L activate L activate n__zeros zeros U11 tt L U12 tt activate L length cons N L U11 tt activate L true length cons N L U11 tt activate L U11 tt L U12 tt activate L U12 tt L length activate L true Failed! NaTT certifiable-1.6