[R] Auxilliary args in gls

[Primer, Jeremy (FID)]

I am trying to fit a B-spline regression model with a corStruct using
gls. I am using bs() and specifying the knots myself. If I make the
knots data-dependent, this works but has undesirable side-effects. I
prefer to reference an auxilliary variable "knots" in my model formula.
It should not be part of the data frame, as it is a vector of a
different length. How can this be done?

The gls() code suggests it may be difficult to keep the auxillary arg
out of the model.frame but in the evaluation frame of the formula:

    glsSt <- glsStruct(corStruct = correlation, varStruct =
varFunc(weights))
    mfArgs <- list(formula = asOneFormula(formula(glsSt), model, 
        groups), data = data, na.action = na.action)
    if (!missing(subset)) {
        mfArgs[["subset"]] <- asOneSidedFormula(Call[["subset"]])[[2]]
    }
    mfArgs$drop.unused.levels <- TRUE
    dataMod <- do.call("model.frame", mfArgs)


Thanks,
Jeremy Primer 
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