code: @SuppressWarnings({"unchecked"})
    public void reduce(Object key, Iterator values, OutputCollector output,
                       Reporter reporter) throws IOException {
      Reducer reducer = chain.getReducer();
      if (reducer != null) {
        reducer.reduce(key, values, chain.getReducerCollector(output, reporter),
                       reporter);
      }
    }
docs: Chains the <code>reduce(...)</code> method of the Reducer with the
  <code>map(...) </code> methods of the Mappers in the chain.