docs: def optimize_var(self):
          """ Optimize the VAR model's hyperparameters (such as regularization).
  
          Returns
          -------
          self : Workspace
              The Workspace object.
  
          Raises
          ------
          RuntimeError
              If the :class:`Workspace` instance does not contain source activations.
          """
          if self.activations_ is None:
              raise RuntimeError("VAR fitting requires source activations (run do_mvarica first)")
  
          self.var_.optimize(self.activations_[self.trial_mask_, :, :])
          return self