code: public void map(DocumentID key, DocumentAndOp value,
        OutputCollector<DocumentID, DocumentAndOp> output, Reporter reporter)
        throws IOException {
      output.collect(key, value);
    }
docs: /* (non-Javadoc)
  @see org.apache.hadoop.mapred.Mapper#map(java.lang.Object, java.lang.Object, org.apache.hadoop.mapred.OutputCollector, org.apache.hadoop.mapred.Reporter)