first
0
X
nil
first
s
X
cons
Y
Z
cons
Y
first
X
Z
from
X
cons
X
from
s
X
2.2
from
X
from
s
X
first
s
X
cons
Y
Z
first
X
Z
true
first
s
X
cons
Y
Z
first
X
Z
true
s
1
1
1
0
0
0
from
1
0
first
2
1
2
0
nil
0
0
first
2
0
from
1
0
cons
2
2
28958
from
X
from
s
X
true
Failed!
NaTT
certifiable-1.6