def _add_mixing_variable_names_to_individual_vars(self):
        """
        Ensure that the model objects mixing variables are added to its list of
        individual variables.
        """
        assert isinstance(self.ind_var_names, list)
        # Note that if one estimates a mixed logit model, then the mixing
        # variables will be added to individual vars. And if one estimates
        # the model again (perhaps from different starting values), then
        # an error will be raised when creating the coefs series because we
        # will have added the mixing variables twice. The condition below
        # should prevent this error.
        already_included = any(["Sigma " in x for x in self.ind_var_names])

        if self.mixing_vars is not None and not already_included:
            new_ind_var_names = ["Sigma " + x for x in self.mixing_vars]
            self.ind_var_names += new_ind_var_names
        return None
Ensure that the model objects mixing variables are added to its list of
        individual variables.