Nested DTDs

[James Tauber]

By "nested DTDs" I assume you mean making use of one DTD's vocabulary with a
document that makes use of another DTD's vocabulary. It is possible if the
DTDs are so designed.

Imagine that you have a "report" DTD in the file report.dtd:

<!ELEMENT report (title,paragraph+)>
<!ELEMENT title (#PCDATA)>
<!ELEMENT paragraph (#PCDATA|emph)*>
<!ELEMENT emph (#PCDATA)>

And you also have a "person" DTD in the file person.dtd:

<!ELEMENT person (firstname, lastname)>
<!ELEMENT firstname (#PCDATA)>
<!ELEMENT lastname (#PCDATA)>

It is easy to make a new DTD that combined the vocabularies in the file
combined.dtd:

<!ENTITY % report "report.dtd">
<!ENTITY % person "person.dtd">
%report;
%person;

Ah, you say, but how do I actually use both of these together? How do I
allow, for example, a person to be marked up in a paragraph? You can't
override the content specification for paragraph so that combined.dtd reads:

<!ENTITY % report "report.dtd">
<!ENTITY % person "person.dtd">
<!ELEMENT paragraph (#PCDATA|person|emph)*>
%report; <!-- this is an error -->
%person;

What you *can* do, or rather what the designer of the report DTD can do, is
write report.dtd as:

<!ELEMENT report (title,paragraph+)>
<!ELEMENT title (#PCDATA)>
<!ENTITY % paragraph.ext "">
<!ELEMENT paragraph (#PCDATA| %para.ext; emph)*>
<!ELEMENT emph (#PCDATA)>

This makes the content model of the paragraph element type extensible.

Because entity declarations can be overridden (by declaring a new literal
value earlier) you can have a combined.dtd:

<!ENTITY % report "report.dtd">
<!ENTITY % person "person.dtd">
<!ENTITY % paragraph.ext "person |">
%report;
%person;

I hope this helps.

James



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