from X cons X from s X after 0 XS XS after s N cons X XS after N XS 2.2 from X from s X after s N cons X XS after N XS true after s N cons X XS after N XS true s 1 1 after 2 0 0 0 0 from 1 0 after 2 2 0 from 1 0 cons 2 2 1 from X from s X true Failed! NaTT certifiable-1.6