code: def _add_mixing_variable_names_to_individual_vars(self):
          
          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
docs: Ensure that the model objects mixing variables are added to its list of
          individual variables.