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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 access_token(self, cookie):
 """Make and store access token as proxy for the access cookie. Create an access token to act as a proxy for access cookie, add it to the dict of accepted access tokens with (cookie, current timestamp) as the value. Return the access token. Return None if cookie is not set. """

        if (cookie):
            token = self._generate_random_string(self.access_tokens)
            self.access_tokens[token] = (cookie, int(time.time()))
            return token
        else:
            return None