[R] syntax for pdDiag (nlme)

[i.m.s.white]

At the top of page 283 of Pinheiro and Bates, a covariance structure for
the indomethicin example is specified as

random = pdDiag(A1 + lrc1 + A2 + lrc2 ~ 1)

The argument to pdDiag() looks like a two-sided formula, and I'm struggling
to reconcile this with the syntax described in Ch4 of the book and online.
Further down page 283 the formula is translated into

list(A1 ~ 1, lrc1 ~ 1, A2 ~ 1, lrc2 ~ 1)

which I find just as puzzling. Can anyone help?

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