docs: def eval(self, x_values):
          """
          Evaluate the ACE regression at any combination of independent variable values.
  
          Parameters
          ----------
          x_values : iterable
              a float x-value for each independent variable, e.g. (1.5, 2.5)
          """
          if len(x_values) != len(self.phi_continuous):
              raise ValueError('x_values must have length equal to the number of independent variables '
                               '({0}) rather than {1}.'.format(len(self.phi_continuous),
                                                               len(x_values)))
  
          sum_phi = sum([phi(xi) for phi, xi in zip(self.phi_continuous, x_values)])
          return float(self.inverse_theta_continuous(sum_phi))