code: @Override
  	protected DataSet getNextDataSetImpl(final String dataSetKey) {
  		for (ExcelFile excelFile : getExcelFiles()) {
  			Map<String, List<Map<String, String>>> data = excelFile.getData();
  			List<Map<String, String>> dataList = data.get(dataSetKey);
  
  			if (dataList != null) {
  				MutableInt counter = dataSetIndices.get(dataSetKey);
  				if (counter == null) {
  					counter = new MutableInt(0);
  					dataSetIndices.put(dataSetKey, counter);
  				}
  
  				if (counter.getValue() >= dataList.size()) {
  					// no more data available
  					return null;
  				}
  
  				// turn data map into a data set
  				Map<String, String> dataMap = dataList.get(counter.getValue());
  				DataSet dataSet = new DefaultDataSet(dataMap);
  
  				// advance the counter for the next dataset
  				counter.increment();
  
  				return dataSet;
  			}
  		}
  		return null;
  	}
docs: Goes through all configured Excel files until data for the specified key is found. All sheets
  of a file a check before the next file is considered.