Example: Replace a base element with the domain extensions

In this scenario, an information architect wants to remove the <ph> element but allow the extensions of <ph> that exist in the highlighting, programming, software, and user interface domains.

The information architect creates an entities file named noPhConstraint.ent, where "no" is a qualifier string that characterizes the constraint.
The information architect adds the following content to noPhConstraint.ent:
```




<!ENTITY ph-constraints     
  "(topic noPh-ph-c)"
>
```
Note: Because the highlighting and programming domains cannot be generalized without the <ph> element, this entity must be defined so that there is a separate parenthetical expression that can be included in the @domains attribute for the topic.

The information architect then integrates the constraint module into a document-type shell for concept by adding the following section above the "TOPIC ELEMENT INTEGRATION" comment:

<!-- ============================================================= -->
<!--                    CONTENT CONSTRAINT INTEGRATION             -->
<!-- ============================================================= -->

<!ENTITY % noPh-ph-c-def  
  PUBLIC "-//ACME//ELEMENTS DITA Ph Constraint//EN" 
  "acme-PhConstraint-constraints" "noPhConstraint.ent">
%noPh-ph-c-def;

In the "DOMAIN EXTENSIONS" section, the information architect removes the reference to the <ph> element:

<!-- Removed "ph | " so as to make <ph> not available, only the domain extensions. -->
<!ENTITY % ph           "%pr-d-ph; |
                         %sw-d-ph; | 
                         %ui-d-ph;
                        ">

She then adds the constraint to the list of domains and constraints that need to be included in the value of the @domains attribute:

<!-- ============================================================= -->
<!--                    DOMAINS ATTRIBUTE OVERRIDE                 -->
<!-- ============================================================= -->

<!ENTITY included-domains 
                          "&concept-att;
                           &hi-d-att; 
                           &indexing-d-att;
                           &pr-d-att; 
                           &sw-d-att;
                           &ui-d-att;
                           &ph-constraint;
  "
>

After updating the catalog.xml file to include the new constraints file, the information architect's work is done.