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