is_empty
nil
true
is_empty
cons
x
l
false
hd
cons
x
l
x
tl
cons
x
l
cons
x
l
append
l1
l2
ifappend
l1
l2
is_empty
l1
ifappend
l1
l2
true
l2
ifappend
l1
l2
false
cons
hd
l1
append
tl
l1
l2
2.2
ifappend
l1
l2
false
append
tl
l1
l2
append
l1
l2
is_empty
l1
ifappend
l1
l2
false
tl
l1
ifappend
l1
l2
false
hd
l1
append
l1
l2
ifappend
l1
l2
is_empty
l1
true
append
l1
l2
ifappend
l1
l2
is_empty
l1
ifappend
l1
l2
false
append
tl
l1
l2
true
Failed!
NaTT
certifiable-1.6