def covariance(self):
from numpy_sugar.linalg import ddot, sum2diag<EOL>Q0 = self._QS[<NUM_LIT:0>][<NUM_LIT:0>]<EOL>S0 = self._QS[<NUM_LIT:1>]<EOL>return sum2diag(dot(ddot(Q0, self.v0 * S0), Q0.T), self.v1)<EOL>
r"""Covariance of the prior.

        Returns
        -------
        :class:`numpy.ndarray`
            :math:`v_0 \mathrm K + v_1 \mathrm I`.