code: def _g(self, z):
          
          return np.exp(np.multiply(-self.theta, z)) - 1
docs: 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