fst
0
Z
nil
fst
s
X
cons
Y
Z
cons
Y
fst
X
Z
from
X
cons
X
from
s
X
add
0
X
X
add
s
X
Y
s
add
X
Y
len
nil
0
len
cons
X
Z
s
len
Z
2.2
len
cons
X
Z
len
Z
add
s
X
Y
add
X
Y
fst
s
X
cons
Y
Z
fst
X
Z
from
X
from
s
X
true
from
X
from
s
X
true
Failed!
NaTT
certifiable-1.6