f true xs f eq s length xs length cons a xs cons a xs length nil 0 length cons x xs s length xs eq 0 0 true eq s x 0 false eq 0 s y false eq s x s y eq x y 2.2 length cons x xs length xs f true xs length cons a xs f true xs length xs f true xs f eq s length xs length cons a xs cons a xs eq s x s y eq x y f true xs eq s length xs length cons a xs true f true xs f eq s length xs length cons a xs cons a xs true Failed! NaTT certifiable-1.6