is_empty
nil
true
is_empty
cons
x
l
false
hd
cons
x
l
x
tl
cons
x
l
l
append
l1
l2
ifappend
l1
l2
l1
ifappend
l1
l2
nil
l2
ifappend
l1
l2
cons
x
l
cons
x
append
l
l2
2.2
append
l1
l2
ifappend
l1
l2
l1
ifappend
l1
l2
cons
x
l
append
l
l2
true
ifappend
l1
l2
cons
x
l
append
l
l2
append
l1
l2
ifappend
l1
l2
l1
true
hd
1
0
append
2
1
1
is_empty
1
0
ifappend
3
0
false
0
0
true
0
0
tl
1
0
append
2
0
tl
1
0
nil
0
0
is_empty
1
0
hd
1
0
cons
2
2
2
ifappend
3
3
0
NaTT
certifiable-1.6