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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 add_user(self, user_name, role='user'):
 """ Calls CF's associate user with org. Valid roles include `user`, `auditor`, `manager`,`billing_manager` """

        role_uri = self._get_role_uri(role=role)
        return self.api.put(path=role_uri, data={'username': user_name})