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Parsing instances and instance parts
Prerequisite Concepts

SGML documents have the property that, context issues aside, any complete subelement of a document instance can be parsed as if it were a full document instance, differing only from parsing the "full" instance in the selection of an appropriate document element for the instance. OmniMark supports full instance and instance part parsing by allowing for an optional program specification of the document-element when doing instance parsing.

Here the phrase "instance part" is used to describe both full instances and complete subelements of instances (a part or a whole).

The primary utility of instance part parsing is expected to be in the context of a database system, in which "instance parts" of SGML documents are kept, for the purpose of independent editing and multiple use. A key aspect of instance part use, and an increasingly important requirement in the documentation industry, is "reusability". A document database really takes on value when its components (instance parts) can be used in multiple contexts. Even where a component may actually be used only once, it is often the case that the context in which it is used, even what book or manual it will finally appear in, is not decided until after most of the components have already been created.

Both reuse and "use after authoring" require that the form of an instance part not depend on the context of its use. Certainly there should be no assumptions about what elements appear around it, or how deep it is in the document structure hierarchy.

These considerations lead one to the point of view that an instance part should not depend on any context other than the DTD itself.

On the other hand, an instance part should be allowed to take advantage of SGML minimization features. Using minimization provides flexibility in how instance parts are created and edited. Those minimization features that have been found most successful, OMITTAG and (most of) SHORTTAG, tend to be independent of context, at least in their successful forms. The minimization features that have been found least successful, RANK and #CURRENT attributes, tend to be most dependent on context. So a context-free approach to instance parts is consistent with using the more successful types of minimization.