Example: Constrain attributes for the <section> element

Example

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 
                '%filter-atts;
                 base      CDATA #IMPLIED
                 %base-attribute-extensions;
                 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
              %conref-atts;
              %select-atts;
              %localization-atts;
              outputclass CDATA   #IMPLIED">

   Note: 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.