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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 _g(self, z):
 """Helper function to solve Frank copula. This functions encapsulates :math:`g_z = e^{-\\theta z} - 1` used on Frank copulas. Argument: z: np.ndarray Returns: np.ndarray """

        return np.exp(np.multiply(-self.theta, z)) - 1