@property<EOL><INDENT>@cache<EOL>def null_beta(self):<DEDENT>
return rsolve(self._MKiM, self._MKiy)<EOL>
Optimal 𝛃 according to the marginal likelihood.

It is compute by solving the equation ::

    MᵀK⁻¹M𝛃 = MᵀK⁻¹𝐲,

for 𝐲 = vec(Y) and M = (A ⊗ X)vec(𝚩).

Returns
-------
effsizes : ndarray
    Optimal 𝛃.