code: private static Collection<String> importSheets(Xml nodeSheets)
      {
          final Collection<Xml> children = nodeSheets.getChildren(NODE_TILE_SHEET);
          final Collection<String> sheets = new ArrayList<>(children.size());
  
          for (final Xml nodeSheet : children)
          {
              final String sheetFilename = nodeSheet.getText();
              sheets.add(sheetFilename);
          }
  
          return sheets;
      }
docs: Import the defined sheets.
  
  @param nodeSheets The sheets node (must not be <code>null</code>).
  @return The sheets filename.