gr() is an older reserved marker for group-level effects with a
user-supplied covariance matrix. New user-facing documentation should prefer
the biological and matrix-specific structured-effect markers: animal() for
pedigree or additive-relatedness animal models, phylo() for phylogenetic
dependence, spatial() for spatial dependence, and relmat() for a
validated user-supplied relatedness or precision matrix. gr() remains
reserved while that clearer surface is completed.