Include the domain element declarations.
In concept.dtd, find the comment that says "DOMAIN ELEMENT INTEGRATION". Following that
comment, add a declaration and reference for the xmlDomain.mod declaration set:
<!ENTITY % xml-d-def
SYSTEM "xmlDomain.mod"
>
%xml-d-def;
As for the .ent file, the actual system identifier needs to reflect the actual
location of your version of xmlDomain.mod.