@property<EOL><INDENT>def beta_covariance(self):<DEDENT>
H = self._terms["<STR_LIT:H>"]<EOL>return inv(H)<EOL>
Estimates the covariance-matrix of the optimal beta.

Returns
-------
beta-covariance : ndarray
    (MᵀK⁻¹M)⁻¹.

References
----------
.. Rencher, A. C., & Schaalje, G. B. (2008). Linear models in statistics. John
   Wiley & Sons.