code: 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;
      }
docs: Generates a blank node identifier for the given key using the algorithm
  specified in:
  
  http://www.w3.org/TR/json-ld-api/#generate-blank-node-identifier
  
  @param id
  The id, or null to generate a fresh, unused, blank node
  identifier.
  @return A blank node identifier based on id if it was not null, or a
  fresh, unused, blank node identifier if it was null.