A function to compute cross-validation errors
Xvalid_all(func, mf, maxL, n.fold, minbucket, minsplit, cp, lookahead)the function used to fit the model
the data set with formula specified
the maximum number of splits
the number of folds
the minimum number of the studies in a terminal node
the minimal number of studies in a parent node to be split
the stopping rule for decrease of between-subgroups Q. Any split that does not decrease the between-subgroups Q is not attempted.
an argument indicating whether to apply the "look-ahead" strategy when fitting the tree