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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 validate_nonce(self, username, iat, nonce):
 """ Confirm that the given nonce hasn't already been used. :param username: Username as a string. :param iat: Unix timestamp float or integer of when the nonce was used. :param nonce: Nonce value. :return: True if nonce is valid, False if it is invalid. """

        key = self.create_nonce_key(username, iat)
        used = cache.get(key, [])
        return nonce not in used