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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 get_access_token(self, verifier=None):
 """ Return the access token for this API. If we've not fetched it yet, go out, request and memoize it. """

        
        if self._access_token is None:
            self._access_token, self._access_token_dict = self._get_access_token(verifier)
        return self._access_token