\Header{update.formula}{Model Updating}
\keyword{models}{update.formula}
\begin{Description}\relax
\code{update.formula} is used to update model formulae.
This typically involves adding or dropping terms,
but updates can be more general.\end{Description}
\begin{Usage}
\begin{verbatim}
update.formula(old, new)
\end{verbatim}
\end{Usage}
\begin{Arguments}
\begin{ldescription}
\item[\code{old}] a model formula to be updated.
\item[\code{new}] a formula giving a template which specifies how to update.
\end{ldescription}
\end{Arguments}
\begin{Details}\relax
The function works by first identifying the \emph{left-hand side}
and \emph{right-hand side} of the \code{old} formula.
It then examines the \code{new} formula and substitutes
the \emph{lhs} of the \code{old} formula for any occurence
of "\code{.}" on the left of \code{new}, and substitutes
the \emph{rhs} of the \code{old} formula for any occurence
of "\code{.}" on the right of \code{new}.\end{Details}
\begin{Value}
The updated formula is returned.\end{Value}
\begin{SeeAlso}\relax
\code{\Link{terms}}, \code{\Link{model.matrix}}.\end{SeeAlso}
\begin{Examples}
\begin{ExampleCode}
update.formula(y ~ x,    ~ . + x2) #> y ~ x + x2
update.formula(y ~ x, log(.) ~ . ) #> log(y) ~ x
\end{ExampleCode}
\end{Examples}

