nthtail
n
l
cond
ge
n
length
l
n
l
cond
true
n
l
l
cond
false
n
l
tail
nthtail
s
n
l
tail
nil
nil
tail
cons
x
l
l
length
nil
0
length
cons
x
l
s
length
l
ge
u
0
true
ge
0
s
v
false
ge
s
u
s
v
ge
u
v
2.2
ge
s
u
s
v
ge
u
v
nthtail
n
l
ge
n
length
l
nthtail
n
l
length
l
nthtail
n
l
cond
ge
n
length
l
n
l
cond
false
n
l
tail
nthtail
s
n
l
length
cons
x
l
length
l
cond
false
n
l
nthtail
s
n
l
true
cond
false
n
l
nthtail
s
n
l
nthtail
n
l
cond
ge
n
length
l
n
l
true
Failed!
NaTT
certifiable-1.6