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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def fit_var(self):
 """ Fit a VAR model to the source activations. 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_.fit(data=self.activations_[self.trial_mask_, :, :])
        self.connectivity_ = Connectivity(self.var_.coef, self.var_.rescov, self.nfft_)
        return self