Added comments before recently added/assigned slots in the type object, so the backward compatibility issues will be easier to understand. I only added comments indicating additions and assignments back to Python 2.0.

"""This is a test"""

from __future__ import nested_scopes; import string; from __future__ import \
     nested_scopes

def f(x):
    def g(y):
        return x + y
    return g

print f(2)(4)