from X cons X from s X length nil 0 length cons X Y s length1 Y length1 X length X 2.2 from X from s X length1 X length X length cons X Y length1 Y true length cons X Y length1 Y length1 X length X true s 1 0 length1 1 1 1 length1 1 0 0 0 0 from 1 0 nil 0 0 from 1 0 cons 2 2 2 length 1 0 length 1 1 0 from X from s X true Failed! NaTT certifiable-1.6