question: public int prune ( double [ ] alphas , double [ ] errors , Instances test ) throws Exception { Vector < LMTNode > nodeList ; CompareNode comparator = new CompareNode ( ) ; treeErrors ( ) ; calculateAlphas ( ) ; nodeList = getNodes ( ) ; boolean prune = ( nodeList . size ( ) > 0 ) ; alphas [ 0 ] = 0 ; Evaluation eval ; if ( errors != null ) { eval = new Evaluation ( test ) ; eval . evaluateModel ( this , test ) ; errors [ 0 ] = eval . errorRate ( ) ; } int iteration = 0 ; while ( prune ) { iteration ++ ; LMTNode nodeToPrune = Collections . min ( nodeList , comparator ) ; nodeToPrune . m_isLeaf = true ; alphas [ iteration ] = nodeToPrune . m_alpha ; if ( errors != null ) { eval = new Evaluation ( test ) ; eval . evaluateModel ( this , test ) ; errors [ iteration ] = eval . errorRate ( ) ; } treeErrors ( ) ; calculateAlphas ( ) ; nodeList = getNodes ( ) ; prune = ( nodeList . size ( ) > 0 ) ; } alphas [ iteration + 1 ] = 1.0 ; return iteration ; }
target:  Method for performing one fold in the cross-validation of the cost-complexity parameter. Generates a sequence of alpha-values with error estimates for the corresponding (partially pruned) trees, given the test set of that fold.