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# Can you explain how an empty list can be passed in long_total and that's not seen as None?

these steps/checks in long_total are a little confusing...

if a is not None and books and books is not None and b is None:

• you need the check 'and books' otherwise the long_total borks when b = books.pop(0) - IndexError: pop from empty list

and later if a is not None and b is None and not books or books is None:

• surely if books is None - there are no books..?
```def long_total(a=None, b=None, books=None):
if a is None and b is None and books is None:
return None
if a is None and b is None and books is not None:
a = books.pop(0) #car
b = books.pop(0) #car(cdr)
return long_total(a, b, books)
if a is not None and books and books is not None and b is None: #need to check books otherwise error trying to pop books(0)
b = books.pop(0) # is you've a returned value
return long_total(a, b, books)
if a is not None and b is not None and books is not None:
return long_total(a+b, None, books) # if you have a product return without b to step above
if a is not None and b is not None and not books:
return long_total(a+b, None, None)
if a is not None and b is None and not books or books is None: