code: private static void exportSheets(Xml nodeSheets, Collection<String> sheets)
      {
          for (final String sheet : sheets)
          {
              final Xml nodeSheet = nodeSheets.createChild(NODE_TILE_SHEET);
              nodeSheet.setText(sheet);
          }
      }
docs: Export the defined sheets.
  
  @param nodeSheets Sheets node (must not be <code>null</code>).
  @param sheets Sheets defined (must not be <code>null</code>).