

   GGeenneerraall CCoorrrreellaattiioonn SSttrruuccttuurree

        corSymm(value, form)

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

      value: an optional vector with the parameter values.
             Default is `numeric(0)', which results in a vector
             of zeros of appropriate dimension being assigned
             to the parameters when `object' is initialized
             (corresponding to an identity correlation struc-
             ture).

       form: a one sided formula of the form `~ t', or code{~ t
             | g}, specifying a time covariate `t' and,
             optionally, a grouping factor `g'. A covariate for
             this correlation structure must be integer valued.
             When a grouping factor is present in `form', the
             correlation structure is assumed to apply only to
             observations within the same grouping level;
             observations with different grouping levels are
             assumed to be uncorrelated. Defaults to `~ 1',
             which corresponds to using the order of the obser-
             vations in the data as a covariate, and no groups.

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

        This function is a constructor for the `corSymm' class,
        representing a general correlation structure. The
        internal representation of this structure, in terms of
        unconstrained parameters, uses the spherical
        parametrization defined in Pinheiro and Bates (1996).
        Objects created using this constructor need to be later
        initialized using the  appropriate `initialize' method.

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

        an object of class `corSymm' representing a general
        correlation structure.

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

        Jose Pinheiro and Douglas Bates

   RReeffeerreenncceess::

        Pinheiro, J.C. and Bates., D.M.  (1996) "Unconstrained
        Parametrizations for Variance-Covariance Matrices",
        Statistics and Computing, 6, 289-296.

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

        `initialize.corSymm'

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

        library(lme)
        ## covariate is observation order and grouping factor is Subject
        cs1 <- corSymm(form = ~ 1 | Subject)

