code: protected Sheet fetchSheet(String sheetName) {
          logger.entering(sheetName);
          Sheet sheet = workBook.getSheet(sheetName);
          if (sheet == null) {
              IllegalArgumentException e = new IllegalArgumentException("Sheet '" + sheetName + "' is not found.");
              throw e;
          }
          logger.exiting(sheet);
          return sheet;
      }
docs: A utility method, which returns {@link Sheet} for a given sheet name.
  
  @param sheetName - A string that represents a valid sheet name.
  @return - An object of type {@link Sheet}