What is a “reference” in SGML/XML?
In six cases for SGML’s “declared value,” where it was said that semantic checking was indeed the role of an SGML parser, the semantic aspect was exceedingly weak. In the case of ID and IDREF(S), the lack of support for relational semantics has come over into XML. How so? The declared value of “ID” involves the semantic of global uniqueness, while the related value “IDREF(S)” implies the nomination of (reference to) one or more of these global IDs . The weakness of this kind of semantics, if it can legitimately be called “semantics,” is revealed in several ways. First, SGML/XML have no mechanism for expressing constraints upon the type of document object that is referenced via the ID-IDREF mechanism. One might expect to be able to declare that a “bibliographic reference” might indeed point from an in-text citation to a bibliographic-reference object (e.g., to an item in a reading list), but the ID-IDREF mechanism cannot support this constraint. In modeling genealogical information, one
