docs: public Long reserveAdd(String transactionId, Data value) {
          if (value != null && getCollection().contains(new CollectionItem(INVALID_ITEM_ID, value))) {
              return null;
          }
          final long itemId = nextId();
          txMap.put(itemId, new TxCollectionItem(itemId, null, transactionId, false));
          return itemId;
      }