docs: @Override
    public Deque<MutablePair<DeltaType, Object>> getByKey(String key) {
      lock.readLock().lock();
      try {
        Deque<MutablePair<DeltaType, Object>> deltas = this.items.get(key);
        if (deltas != null) {
          // returning a shallow copy
          return new LinkedList<>(deltas);
        }
      } finally {
        lock.readLock().unlock();
      }
      return null;
    }