accumulator generators

I was reading this <a href="this http://www.paulgraham.com/icad.html";>Paul
Graham article</a> and he builds an accumuator generator function in
the appendix. His looks like this:

<pre>
def foo(n):
s = [n]
def bar(i):
s[0] += i
return s[0]
return bar
</pre>

Why does that work, but not this:

<pre>
def foo(n):
s = n
def bar(i):
s += i
return s
return bar
</pre>
.

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