Internal function used compute the products
`(X otimes Theta)^t (I otimes Sigma^{-1}) (X otimes Theta)`
and
`(X otimes Theta)^t (I otimes Sigma^{-1}) (y)`
in cross-sectional VB algorithm and Gibbs sampler
Usage
Xt_siginv_X(tx, siginv, y = NULL)
Arguments
tx
transpose of the X design matrix
siginv
inverse variance matrix
y
outcome matrix. if NULL, function computes
first product; if not, function computes second product.