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varFixed class,
representing a variance function with fixed variances. Letting $v$
denote the variance covariate defined in value, the variance
function $\sigma^2(v)$ for this class is
$\sigma^2(v)=|v|$. The variance covariate $v$ is
evaluated once at initialization and remains fixed thereafter. No
coefficients are required to represent this variance function.varFixed(value)~ v specifying a
variance covariate v. Grouping factors are ignored.varFixed object representing a fixed variance function
structure, also inheriting from class varFunc.varClasses,
varWeights.varFunc,
varFunc