# Model functions

## and() function

and(`t,r`)
and
a(`t,r`)

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
and ` t`
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

male and(female)
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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