XML in a Nutshell, 3rd Edition

Redefining Parameter Entities

What makes parameter entity references particularly powerful is that they can be redefined. If a document uses both internal and external DTD subsets, then the internal DTD subset can specify new replacement text for the entities. If ELEMENT and ATTLIST declarations in the external DTD subset are written indirectly with parameter entity references, instead of directly with literal element names, the internal DTD subset can change the DTD for the document. For instance, a single document could add a bedrooms child element to the listings by redefining the residential_content entity like this:

<!ENTITY % residential_content "address, footage, rooms,
                                bedrooms, baths, available_date">

In the event of conflicting entity declarations, the first one encountered takes precedence. The parser reads the internal DTD subset first. Thus, the internal definition of the residential_content entity is used. When the parser reads the external DTD subset, every declaration that uses the residential_content entity will contain a bedrooms child element it wouldn’t otherwise have.

Modular XHTML, which we’ll discuss in Chapter 7, makes heavy use of this technique to allow particular documents to select only the subset of HTML that they actually need.