f
true
xs
f
isList
xs
append
cons
a
nil
xs
isList
nil
true
isList
cons
x
xs
isList
xs
append
nil
ys
ys
append
cons
x
xs
ys
cons
x
append
xs
ys
2.2
isList
cons
x
xs
isList
xs
f
true
xs
append
cons
a
nil
xs
f
true
xs
f
isList
xs
append
cons
a
nil
xs
append
cons
x
xs
ys
append
xs
ys
f
true
xs
isList
xs
true
f
true
xs
f
isList
xs
append
cons
a
nil
xs
true
Failed!
NaTT
certifiable-1.6