docs: protected void useVariableForCustomExpression(JRDesignGroup group,
  			JRDesignExpression jrExpression, CustomExpression customExpression)
  			throws JRException {
  		//1) Register CustomExpression object as a parameter
  		String expToGroupByName = group.getName() + "_custom_expression";
  		registerCustomExpressionParameter(expToGroupByName, customExpression);
  		
  		//2) Create a variable which is calculated through the custom expression
  		JRDesignVariable gvar = new JRDesignVariable();
  		String varName = group.getName() + "_variable_for_group_expression";
  		gvar.setName(varName);
  		gvar.setCalculation( CalculationEnum.NOTHING );
  		gvar.setValueClassName(customExpression.getClassName());
  		
  		String expText = ExpressionUtils.createCustomExpressionInvocationText(customExpression, expToGroupByName, false);
  		JRDesignExpression gvarExp = new JRDesignExpression();
  		gvarExp.setValueClassName(customExpression.getClassName());
  		gvarExp.setText(expText);
  		gvar.setExpression(gvarExp);
  		getDjd().addVariable(gvar);
  		
  		//3) Make the group expression point to the variable
  		jrExpression.setText("$V{"+varName+"}");
  		jrExpression.setValueClassName(customExpression.getClassName());
  		log.debug("Expression for CustomExpression usgin variable = \"" + varName + "\" which point to: " + expText);
  	}