Example: Restrict the content model for the <topic> element using DTD

In this scenario, the DITA architect for Acme Incorporated wants to redefine the content model for the topic document type. They want to omit certain elements, make the <shortdesc> element required, and disallow topic nesting.

Specifically, the DITA architect wants to redefine the content model in the following ways:

• Remove <abstract>
• Require <shortdesc>
• Remove <related-links>
• Remove the %task-info-types; entity in order to disallow topic nesting

1. The DITA architect creates a constraint module: acme-TopicConstraint.mod.
2. They add the following content to acme-TopicConstraint.mod:

   
   <!-- ============================================================= -->
   <!--                    CONSTRAINED TOPIC ENTITIES                 -->
   <!-- ============================================================= -->
   
   <!-- Declares the entities referenced in the constrained content  -->
   <!-- model.                                                       -->
   
   <!ENTITY % title            "title">
   <!ENTITY % shortdesc        "shortdesc">
   <!ENTITY % prolog           "prolog">
   <!ENTITY % body             "body">
   
   <!-- Defines the constrained content model for <topic>.            -->
   
   <!ENTITY % topic.content
                          "((%title;),
                            (%shortdesc;), 
                            (%prolog;)?, 
                            (%body;)?)"
   >

3. They add the constraint module to the catalog.xml file.
4. They then integrate the constraint module into the document-type shell for topic by adding the following content to the "Element-Type Configuration Integration section:

   <!-- ============================================================= -->
   <!--           ELEMENT-TYPE CONFIGURATION INTEGRATION              -->
   <!-- ============================================================= -->
   
   <!ENTITY % topic-constraints-c-def  
     PUBLIC "-//ACME//ELEMENTS DITA Topic Constraint//EN" 
     "acme-TopicConstraint.mod">
   %topic-constraints-c-def;

5. They check their test topic to ensure that the content model is modified as expected.