How does this code works:

[vch]

Here's an example from some book:

def foo(n):
	s = [n]
	def bar(i):
  		s[0] += i
  		return s[0]
  	return bar

what I don't understand is how this example works, taking into account 
the LGB rule. I thought that s is not accessible from bar, but it is, 
apparently. Why?