Example: Redefine the content model for the <topic> element

In this scenario, an information architect for Acme, Incorporated wants to redefine the content model for the topic document type. She wants to omit the <abstract> element and make the <shortdesc> element required; she also wants to omit the <related-links> element and disallow topic nesting.
  1. She creates a .mod file using the following naming conventions: qualiferTagnameConstraint.mod, where qualifer is a string the describes the constraint, and Tagname is the element type name with an initial capital. Her contraint module is named acme-TopicConstraint.mod.
  2. She adds the following content to acme-TopicConstraint.mod:
    <!-- ============================================================= -->
    <!--                    CONSTRAINED TOPIC ENTITIES                 -->
    <!-- ============================================================= -->
    <!-- Declares the entity for the constraint module and defines     -->
    <!-- its contribution to the @domains attribute.                   -->
    <!ENTITY topic-constraints     
      "(topic basic-Topic-c)"
    <!-- Declares the entities referenced in the constrained content  -->
    <!-- model.                                                       -->
    <!ENTITY % title            "title">
    <!ENTITY % titlealts        "titlealts">
    <!ENTITY % shortdesc        "shortdesc">
    <!ENTITY % prolog           "prolog">
    <!ENTITY % body             "body">
    <!-- Defines the constrained content model for <topic>.            -->
    <!ENTITY % topic.content
  3. She then integrates the constraint module into her document-type shell for topic by adding the following section above the "TOPIC ELEMENT INTEGRATION" comment:
    <!-- ============================================================= -->
    <!--                    CONTENT CONSTRAINT INTEGRATION             -->
    <!-- ============================================================= -->
    <!ENTITY % topic-constraints-c-def  
      PUBLIC "-//ACME//ELEMENTS DITA Topic Constraint//EN" 
  4. She then adds the constraint to the list of domains and constraints that need to be included in the value of the @domains attribute for <topic>:
    <!-- ============================================================= -->
    <!--                    DOMAINS ATTRIBUTE OVERRIDE                 -->
    <!-- ============================================================= -->
    <!ENTITY included-domains 
  5. After updating the catalog.xml file to include the new constraints file, her work is done.

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