```
def neumann_set( n ):
if n == 0:
return set()
else:
return neumann_set(n-1).add( neumann_set(n-1) )
```

How do I retrieve Neumann_set(n-1)

```
def neumann_set( n ):
if n == 0:
return set()
else:
return neumann_set(n-1).add( neumann_set(n-1) )
```

How do I retrieve Neumann_set(n-1)

Heat Jack asked:

“How do I retrieve Neumann_set(n-1)”

You call `neumann_set(n-1)`

. You need to make sure n has a value, of course.

```
n = 15
result = neumann_set(n-1) # like neumann_set(14)
```

But your code won’t work because sets are not hashable, so your sets cannot contain sets. It will work for n=0:

```
neumann_set(0) # correctly returns an empty set
```

but for any value above 0, it will fail with a TypeError.

If you explain what a Neumann set is, perhaps we can help you program it correctly.