code: @Override
      public boolean addRef(Object object)
          throws IOException
      {
          if (_isUnshared) {
              _refCount++;
              return false;
          }
  
          int newRef = _refCount;
  
          int ref = addRef(object, newRef, false);
  
          if (ref != newRef) {
              writeRef(ref);
  
              return true;
          }
          else {
              _refCount++;
  
              return false;
          }
      }
docs: If the object has already been written, just write its ref.
  
  @return true if we're writing a ref.