U11 tt U12 splitAt N XS U12 pair YS ZS pair cons X ZS and tt X tail cons N XS afterNth N XS snd splitAt N XS fst pair X Y X head cons N N natsFrom N cons N sel N XS head afterNth N XS snd pair X Y Y splitAt 0 XS pair nil XS splitAt s N cons X U11 tt take N XS fst splitAt N XS 2.2 NaTT certifiable-1.6