[Tutor] Why does this NameError appear?

[Nathan Pinno]

Here is the code:
lista = [x0, x1, x2, x3]
listb = [a0, a1, a2, a3]
print lista
print listb
print match (lista, listb)

And here is the error:
Traceback (most recent call last):
  File "D:\Python22\matches.py", line 1, in ?
    lista = [x0, x1, x2, x3]
NameError: name 'x0' is not defined

Thanks in advance,
Nathan Pinno
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