docs: public CRDTTimestampedLong getAndAdd(long delta, VectorClock observedTimestamps) {
          checkSessionConsistency(observedTimestamps);
          stateWriteLock.lock();
          try {
              checkNotMigrated();
              if (delta < 0) {
                  return getAndSubtract(-delta, observedTimestamps);
              }
              return getAndUpdate(delta, observedTimestamps, true);
          } finally {
              stateWriteLock.unlock();
          }
      }