

   VVaarriiaannccee FFuunnccttiioonn SSttrruuccttuurree

        varFunc(object)

   AArrgguummeennttss::

     object: either an one-sided formula specifying a variance
             covariate, or an object inheriting from class
             code{varFunc}, representing a variance function
             structure.

   DDeessccrriippttiioonn::

        If `object' is a one-sided formula, it is used as the
        argument to `varFixed' and the resulting object is
        returned. Else, if `object' inherits from class `var-
        Func', it is returned unchanged.

   VVaalluuee::

        an object from class `varFunc', representing a variance
        function structure.

   AAuutthhoorr((ss))::

        Jose Pinheiro and Douglas Bates

   SSeeee AAllssoo::

        `varFixed', `varWeights.varFunc', `coef.varFunc'

   EExxaammpplleess::

        vf1 <- varFunc(~age)

