Example: Constrain attributes for the <section> element

In this scenario, an information architect wants to redefine the attributes for the <section> element. He wants to make the @id attribute required and omit the @spectitle attribute.
  1. He creates a .mod file named idRequiredSectionContraint.mod, where "idRequired" is a string that characterizes the constraint.
  2. He adds the following content to idRequiredSectionContraint.mod:
    <!-- ============================================================= -->
    <!--                    CONSTRAINED TOPIC ENTITIES                 -->
    <!-- ============================================================= -->
    <!ENTITY section-constraints     
      "(topic idRequired-section-c)"
    <!-- Declares the entities referenced in the constrained content  -->
    <!-- model.                                                       -->
    <!ENTITY % conref-atts 
                 'conref    CDATA #IMPLIED
                  conrefend CDATA #IMPLIED
                  conaction (mark|pushafter|pushbefore|pushreplace|-dita-use-conref-target) #IMPLIED
                  conkeyref CDATA #IMPLIED' >
    <!ENTITY % filter-atts
                 'props      CDATA #IMPLIED
                  platform   CDATA #IMPLIED
                  product    CDATA #IMPLIED
                  audience   CDATA #IMPLIED
                  otherprops CDATA #IMPLIED
                  %props-attribute-extensions;' >
    <!ENTITY % select-atts 
                  base      CDATA #IMPLIED
                  importance (default|deprecated|high|low|normal|obsolete|optional| 
                              recommended|required|urgent|-dita-use-conref-target) #IMPLIED
                  rev       CDATA #IMPLIED
                  status     (changed|deleted|unchanged|-dita-use-conref-target) #IMPLIED' >
    <!ENTITY % localization-atts 
                 'translate (no|yes|-dita-use-conref-target) #IMPLIED
                  xml:lang CDATA #IMPLIED
                  dir      (lro|ltr|rlo|rtl|-dita-use-conref-target) #IMPLIED' >
    <!-- Declares the constrained content model. Original definition   -->
    <!-- included %univ-atts;, spectitle, and outputclass; now includes-->
    <!-- individual pieces of univ-atts, to make ID required.          -->
    <!ENTITY % section.attributes 
              "id          CDATA   #REQUIRED
               outputclass CDATA   #IMPLIED">


    The information architect had to declare all the parameter entities that are referenced in the redefined attributes for <section>. If he did not do so, none of the attributes that are declared in the conref-atts, select-atts, or localization-atts parameter entities would be available on the <section> element. Furthermore, since the select-atts parameter entity references the filter-atts parameter entity, the filter-atts must be declared and it must precede the declaration for the select-atts parameter entity. The props-attribute-extensions and base-attribute-extensions parameter entities do not need to be declared in the constraint module, because they are declared in the document-type shells before the inclusion of the constraint module.
  3. He then integrates the constraint module into the applicable document-type shells and adds it to his catalog.xml file.

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