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