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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:

def last_modified(self):
        '''Read the last-modified header as a datetime, if present.'''
        lm = self.response.headers.get('last-modified', None)
        return datetime.strptime(lm, '%a, %d %b %Y %H:%M:%S GMT') if lm \
            else None