code: public boolean addRef(Object object)
          throws IOException
      {
          if (_refs == null)
              _refs = new IdentityHashMap();
  
          Integer ref = (Integer) _refs.get(object);
  
          if (ref != null) {
              int value = ref.intValue();
  
              writeRef(value);
              return true;
          }
          else {
              _refs.put(object, new Integer(_refs.size()));
  
              return false;
          }
      }
docs: If the object has already been written, just write its ref.
  
  @return true if we're writing a ref.