/**
  * Evaluate the expression of a specific duration.
  *
  * @param idx the index of the expression
  * @param n the node
  * @return the estimation of the expression
  * @throws RecognitionException if an error occurs during the evaluation
  */
 private int evaluate(int idx, Node n) throws RecognitionException {
   ANTLRDurationEvaluatorParser p = new ANTLRDurationEvaluatorParser(this.read(idx));
   return p.evaluate(n);
 }
 /**
  * Evaluate the expression of a specific duration.
  *
  * @param idx the index of the expression
  * @param vm the virtual machine
  * @return the estimation of the expression
  * @throws RecognitionException if an error occurs during the evaluation
  */
 private int evaluate(int idx, VirtualMachine vm) throws RecognitionException {
   ANTLRDurationEvaluatorParser p = new ANTLRDurationEvaluatorParser(this.read(idx));
   return p.evaluate(vm);
 }