formula(object) includes a "." term, representing
a fitted object, the variance covariate needs to be updated upon
completion of an optimization cycle (in which the variance function
weights are kept fixed). This method function allows a reevaluation of
the variance covariate using the current fitted object and,
optionally, other variables in the original data.update(object, data)varFunc,
representing a variance function structure."." with the current
version of the fitted object (from which fitted values, coefficients,
and residuals can be extracted) and, if necessary, other variables
used to evaluate the variance covariate(s)formula(object) includes a "." term, an
varFunc object similar to object, but with the
variance covariate reevaluated at the current fitted object value;
else object is returned unchanged.needUpdate, covariate<-.varFunc