A significant flaw may have been noticed in the final structure we
arrived at in the last section - when building documents using this
DTD, there's no place to add a <Customer> element. It's not the
root element of the document, and it doesn't appear in any of the
element content models of any of the other elements in the structure.
This is because it is only pointed to, not contained.
Elements that turn out to only be referenced by IDREF(S) need to
be added as allowable element content to the root element of the DTD.
Then, when creating the document, the orphaned elements are created
within the root element and then pointed to, where appropriate.
Applying this rule to our example, we see that we are missing the
<Customer> and <Part> elements. Adding these as allowable
structural content to our root element gives us:
<!ELEMENT SalesData (Invoice*, Customer*, Part*,
MonthlyTotal*)>
<!ATTLIST SalesData
Status (NewVersion | UpdatedVersion | CourtesyCopy) #REQUIRED>
<!ELEMENT Invoice (LineItem*)>
...
Rule 10: Add Missing Elements.
For any element that is only pointed to in the structure created
so far, add that element as allowable element content of the root
element. Set the cardinality suffix of the element being added to
*.
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