Misunderstanding of Eq. 4 in Singer’s (1998) SAS PROC MIXED paper

Singer (1998, p. 327, Eq. 4) gave a big covariance matrix as the following --
...if we combine the variance components for the two random effects together into a single matrix, we would find a highly structured block diagonal matrix. For example, if there were three students in each class, we would have:

If the number of [...]

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