code: func (n *NetStore) FetchFunc(ctx context.Context, ref Address) func(context.Context) error {
  	chunk, fetch, _ := n.get(ctx, ref)
  	if chunk != nil {
  		return nil
  	}
  	return func(ctx context.Context) error {
  		_, err := fetch(ctx)
  		return err
  	}
  }
docs: // FetchFunc returns nil if the store contains the given address. Otherwise it returns a wait function,
  // which returns after the chunk is available or the context is done