a=
"

A

B
"

#xD #xD A #xA #xA B #xD #xA

#xD #xD A #xA #xA B #xD #xA

3.4 Conditional Sections

[Definition: Conditional sections are portions of the document type declaration external subset or of external parameter entities which are included in, or excluded from, the logical structure of the DTD based on the keyword which governs them.]

Conditional Section

Like the internal and external DTD subsets, a conditional section may contain one or more complete declarations, comments, processing instructions, or nested conditional sections, intermingled with white space.

If the keyword of the conditional section is INCLUDE, then the contents of the conditional section MUST be processed as part of the DTD. If the keyword of the conditional section is IGNORE, then the contents of the conditional section MUST NOT be processed as part of the DTD. If a conditional section with a keyword of INCLUDE occurs within a larger conditional section with a keyword of IGNORE, both the outer and the inner conditional sections MUST be ignored. The contents of an ignored conditional section MUST be parsed by ignoring all characters after the "[" following the keyword, except conditional section starts "<![" and ends "]]>", until the matching conditional section end is found. Parameter entity references MUST NOT be recognized in this process.

If the keyword of the conditional section is a parameter-entity reference, the parameter entity MUST be replaced by its content before the processor decides whether to include or ignore the conditional section.

An example:

<!ENTITY % draft 'INCLUDE' >
<!ENTITY % final 'IGNORE' >

<![%draft;[
<!ELEMENT book (comments*, title, body, supplements?)>
]]>
<![%final;[
<!ELEMENT book (title, body, supplements?)>
]]>

4 Physical Structures

[Definition: An XML document may consist of one or many storage units. These are called entities; they all have content and are all (except for the document entity and the external DTD subset) identified by entity name.] Each XML document has one entity called the document entity, which serves as the starting point for the XML processor and may contain the whole document.

Entities may be either parsed or unparsed. [Definition: The contents of a parsed entity are referred to as its replacement text; this text is considered an integral part of the document.]

[Definition: An unparsed entity is a resource whose contents may or may not be text, and if text, may be other than XML. Each unparsed entity has an associated notation, identified by name. Beyond a requirement that an XML processor make the identifiers for the entity and notation available to the application, XML places no constraints on the contents of unparsed entities.]

Parsed entities are invoked by name using entity references; unparsed entities by name, given in the value of ENTITY or ENTITIES attributes.

[Definition: General entities are entities for use within the document content. In this specification, general entities are sometimes referred to with the unqualified term entity when this leads to no ambiguity.] [Definition: Parameter entities are parsed entities for use within the DTD.] These two types of entities use different forms of reference and are recognized in different contexts. Furthermore, they occupy different namespaces; a parameter entity and a general entity with the same name are two distinct entities.