

   MMooddeell UUppddaattiinngg

        update.formula(old, new)

   AArrgguummeennttss::

        old: a model formula to be updated.

        new: a formula giving a template which specifies how to
             update.

   DDeessccrriippttiioonn::

        `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.

   SSeeee AAllssoo::

        `terms', `model.matrix'.

