This method function extracts the covariate(s) associated with the
variance function represented by
object, if any is present.
an object inheriting from class
an optional formula specifying the covariate to be
some methods for this generic require a
object has a
covariate attribute, its value is
NULL is returned.
Jos<c3><a9> Pinheiro and Douglas Bates firstname.lastname@example.org
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