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