Store the trace of stochastics for the computation of the covariance.
        This trace is completely independent from the backend used by the
        sampler to store the samples.

def internal_tally(self):
        """Store the trace of stochastics for the computation of the covariance.
        This trace is completely independent from the backend used by the
        sampler to store the samples."""
        chain = []
        for stochastic in self.stochastics:
            chain.append(np.ravel(stochastic.value))
        self._trace.append(np.concatenate(chain))