code: @SuppressWarnings("unchecked") // no static type for the slot this sits in
    public void accept(CompositeRecordReader.JoinCollector i, K key)
        throws IOException {
      vjoin.clear();
      if (0 == cmp.compare(key, khead)) {
        do {
          vjoin.add(vhead);
        } while (next() && 0 == cmp.compare(key, khead));
      }
      i.add(id, vjoin);
    }
docs: JoinCollector comes from parent, which has