f
a
empty
g
a
empty
f
a
cons
x
k
f
cons
x
a
k
g
empty
d
d
g
cons
x
k
d
g
k
cons
x
d
2.2
f
a
empty
g
a
empty
g
cons
x
k
d
g
k
cons
x
d
f
a
cons
x
k
f
cons
x
a
k
true
f
a
cons
x
k
f
cons
x
a
k
true
f
2
0
f
2
2
0
g
2
0
cons
2
2
1
empty
0
0
g
2
0
f
a
empty
g
a
empty
false
g
cons
x
k
d
g
k
cons
x
d
true
f
2
0
f
2
0
g
2
1
0
cons
2
2
1
empty
0
0
g
2
0
NaTT
certifiable-1.6