docs: String generateBlankNodeIdentifier(String id) {
          if (id != null && blankNodeIdentifierMap.containsKey(id)) {
              return blankNodeIdentifierMap.get(id);
          }
          final String bnid = "_:b" + blankNodeCounter++;
          if (id != null) {
              blankNodeIdentifierMap.put(id, bnid);
          }
          return bnid;
      }