code: public void writeSuperNode(ObjectOutput out) throws IOException {
      if(!isSuperNode()) {
        throw new IllegalStateException("Cannot write as non-super node via writeSuperNode()");
      }
      // write header
      writeExternal(out);
      for(Entry entry : entries) {
        if(entry == null) {
          break;
        }
        entry.writeExternal(out);
      }
    }
docs: This node is a supernode and we only write it to file in order to be able
  to re-load it in another session. It can be loaded by
  {@link #readSuperNode}.
  
  @param out object output