code: public void add(ComposableRecordReader<K,? extends V> rr) throws IOException {
      kids[rr.id()] = rr;
      if (null == q) {
        cmp = WritableComparator.get(rr.createKey().getClass());
        q = new PriorityQueue<ComposableRecordReader<K,?>>(3,
            new Comparator<ComposableRecordReader<K,?>>() {
              public int compare(ComposableRecordReader<K,?> o1,
                                 ComposableRecordReader<K,?> o2) {
                return cmp.compare(o1.key(), o2.key());
              }
            });
      }
      if (rr.hasNext()) {
        q.add(rr);
      }
    }
docs: Add a RecordReader to this collection.
  The id() of a RecordReader determines where in the Tuple its
  entry will appear. Adding RecordReaders with the same id has
  undefined behavior.