private Optional<List<Snak>> processGDMQualifiedAttributes(final Statement statement) {

    final List<Snak> snakList = new ArrayList<>();

    final Optional<Snak> optionalConfidence =
        processGDMQualifiedAttribute(
            CONFIDENCE_QUALIFIED_ATTRIBUTE_IDENTIFIER, statement.getConfidence());
    final Optional<Snak> optionalEvidence =
        processGDMQualifiedAttribute(
            EVIDENCE_QUALIFIED_ATTRIBUTE_IDENTIFIER, statement.getEvidence());
    final Optional<Snak> optionalOrder =
        processGDMQualifiedAttribute(ORDER_QUALIFIED_ATTRIBUTE_IDENTIFIER, statement.getOrder());

    // D:SWARM statement uuid
    final Optional<Snak> optionalUUID =
        processGDMQualifiedAttribute(
            STATEMENT_UUID_QUALIFIED_ATTRIBUTE_IDENTIFIER, statement.getUUID());

    addToSnakList(optionalConfidence, snakList);
    addToSnakList(optionalEvidence, snakList);
    addToSnakList(optionalOrder, snakList);
    addToSnakList(optionalUUID, snakList);

    if (snakList.isEmpty()) {

      return Optional.empty();
    }

    return Optional.of(snakList);
  }
  private static String printGDMStatement(final Statement statement) {

    final StringBuilder sb = new StringBuilder();

    final Long id = statement.getId();

    sb.append("{statement: id ='");

    if (id != null) {

      sb.append(id);
    } else {

      sb.append("no statement id available");
    }

    sb.append("' :: ");

    final String uuid = statement.getUUID();

    sb.append("uuid = '");

    if (uuid != null) {

      sb.append(uuid);
    } else {

      sb.append("no uuid available");
    }

    sb.append("' :: ");

    final String subject = printGDMNode(statement.getSubject());

    sb.append("subject = '").append(subject).append("' :: ");

    final String predicateURI = statement.getPredicate().getUri();

    sb.append("predicate = '").append(predicateURI).append("' :: ");

    final String object = printGDMNode(statement.getObject());

    sb.append("object = '").append(object).append("'}");

    return sb.toString();
  }