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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_nonce(self, url):
 """ Get a nonce to use in a request, removing it from the nonces on hand. """

        action = LOG_JWS_GET_NONCE()
        if len(self._nonces) > 0:
            with action:
                nonce = self._nonces.pop()
                action.add_success_fields(nonce=nonce)
                return succeed(nonce)
        else:
            with action.context():
                return (
                    DeferredContext(self.head(url))
                    .addCallback(self._add_nonce)
                    .addCallback(lambda _: self._nonces.pop())
                    .addCallback(tap(
                        lambda nonce: action.add_success_fields(nonce=nonce)))
                    .addActionFinish())