model.frame method for rpart objects.## S3 method for class 'rpart':
model.frame(formula, \dots)rpart fit.model.frame suitable for fitting an
rpart-based model. Called by rpart and other
functions for manipulating rpart objects.model.frame for
class rpart. It can be invoked by calling model.frame
for an object of the appropriate class, or directly by calling
model.frame.rpart regardless of the class of the object.model.frame