[R] how to personalize split function in rpart

[Simone Vantini]

Hallo!
I am a student of the Politecnico di Milano (Milan, italy) and I'm working
on CARTs. I'm trying to use the R rpart function with a personalized splitfunction... but I'm not able  to do it!
More precisely, I would like to know what is the meaning of the function
'init', 'split' and 'eval' named in the help page.I can't find any answer
in 'Classification and regression trees' (Breiman, ...) , in 'Modern
applied statistics with S-plus' (Venables, Ripley) or even in the on-line
forum.Thank You very much
Simone Vantini