Two questions

        B - cross-validation of rpart trees : a colleague is using S to
generate decision tree and mentioned to me the use of cross-validation. Is
this function enabled in R ? if so, how should one proceed to do it?

Well documented in the rpart manuals and rpart help files.  If you are 
using R version 1.30 or higher, under Windows rpart gets installed as part 
of the recommended package distribution.

 >?xpred.rpart

might get you started.

Also

 >?rpart


-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-.-
r-help mailing list -- Read http://www.ci.tuwien.ac.at/~hornik/R/R-FAQ.html
Send "info", "help", or "[un]subscribe"
(in the "body", not the subject !)  To: r-help-request at stat.math.ethz.ch
_._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._._