[1] RDF Schema 1.1 ( (2014-02-27 21:42:23 +09:00 版)) <https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-schema/index.html#h4_ch_object>

[2] RDF 1.1 Concepts and Abstract Syntax ( (2014-04-26 05:28:00 +09:00 版)) <https://dvcs.w3.org/hg/rdf/raw-file/default/rdf-concepts/index.html#dfn-object>