update.formula(old, new)
old
| a model formula to be updated. |
new
| a formula giving a template which specifies how to update. |
update.formula is used to update model formulae.
This typically involves adding or dropping terms,
but updates can be more general.
The function works by first identifying the left-hand side
and right-hand side of the old formula.
It then examines the new formula and substitutes
the lhs of the old formula for any occurence
of "." on the left of new, and substitutes
the rhs of the old formula for any occurence
of "." on the right of new.
The updated formula is returned.
terms, model.matrix.