/// <summary>
 /// Convert periodic type in ast node to the type in model
 /// </summary>
 /// <param name="periodicTypeAst"></param>
 /// <returns></returns>
 private PeriodicType PeriodicTypeConvert(PeriodicTypeAst periodicTypeAst)
 {
     switch (periodicTypeAst)
     {
         case  PeriodicTypeAst.Day:
             return PeriodicType.Day;
         case PeriodicTypeAst.Week:
             return PeriodicType.Week;
         case PeriodicTypeAst.Month:
             return PeriodicType.Month;
         default:
             throw new ArgumentException("Unexpected periodic type from AST node: " + periodicTypeAst.ToString());
     }
 }