overlay (add) r times the design matrix for model
term t to the existing design matrix.
Specifically, if the model up to this point has p effects
has a effects,
the a columns of the design matrix for t are multiplied by the scalar r
(default value 1.0) and added to the last a of the p columns already defined.
The overlaid term must agree in size with the term it overlays.
This can be used to force a correlation of 1 between two terms as in a diallel analysis
assuming the i th male is the same individual
as the i th female.
Note that if the overlaid term is complex, it must be predefined; e.g.
Tr.male -Tr.female and(Tr.female).
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