docs: @Override
      public void rollback() throws BackendException {
          Throwable excep = null;
          for (IndexTransaction itx : indexTx.values()) {
              try {
                  itx.rollback();
              } catch (Throwable e) {
                  excep = e;
              }
          }
          storeTx.rollback();
          if (excep!=null) { //throw any encountered index transaction rollback exceptions
              if (excep instanceof BackendException) throw (BackendException)excep;
              else throw new PermanentBackendException("Unexpected exception",excep);
          }
      }