code: private Object set(Commit<SetCommand> commit) {
      try {
        Commit<SetCommand> previous = value;
        value = commit;
        if (previous != null) {
          Object result = previous.operation().value();
          previous.close();
          return result;
        }
        return null;
      } catch (Exception e) {
        commit.close();
        throw e;
      }
    }
docs: Sets the value.