SIJIN CHENG; OLAF HARTIG. OPT+: A Monotonic Alternative to OPTIONAL in SPARQL. Journal of Web Engineering, [S. l.], v. 18, n. 1-3, 2019. Disponível em: https://journals.riverpublishers.com/index.php/JWE/article/view/3179. Acesso em: 31 may. 2026.