[Rd] Extending R formulas

[Mathieu Ribatet]

    Dear list,

I'm currently trying to write a new R package to model spatial extremes. 
In particular, for a model fitting procedure, I'd like to use flexible 
response surfaces (like linear models, splines, ...) for the parameters 
of my model. Following this idea, I'd like to allow "a new 
interpretation" of an R formula  given by the user, like y ~ x1 + rb(x2, 
knots, degree, penalty) where x1, x2 are covariates and rb stands for 
using a radial basis smooth splines with given knots, degree and penalty 
coefficient.
Such "new formula" should then provide the appropriate design matrix.

I know that existing packages have done this way too - e.g. mgcv; but 
their codes are quite hard to understand - for me I mean.
Does anyone can provide some useful guidelines to do this kind of stuffs 
or provide more readable codes?

In advance, thank you
Best,
Mathieu

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