Apple open-sources FoundationDB(foundationdb.org)

  @fdb.transactional
  def compare_and_set( tr, key, vold, vnew ):
    if tr[key] == vold:
      tr[key] = vnew

    create table counters(counter int);

    insert into counters(counter) values(1);

    BEGIN TRANSACTION ISOLATION LEVEL serializable; 
    select sum(counter) from counters; 
    /* insert sum into counters. wait until committing next transaction before executing the insert */
    insert into counters(counter) values(1);
    COMMIT;

    /* this transaction should commit before doing the insert in the above transaction and after the above transaction has calculated the sum */
    BEGIN TRANSACTION ISOLATION LEVEL serializable; 
    insert into counters(counter) values(10);
    COMMIT;