code: func NewAttachable(stores map[string]content.Store) session.Attachable {
  	store := &attachableContentStore{stores: stores}
  	service := contentserver.New(store)
  	a := attachable{
  		service: service,
  	}
  	return &a
  }
docs: // NewAttachable creates session.Attachable from aggregated stores.
  // A key of the store map is an ID string that is used for choosing underlying store.