An example
In the DTD subset we write:
 <!ENTITY % TEI.prose "INCLUDE">
 <!ENTITY % biblStruct "IGNORE">
In the prose tagset it says:
<!ENTITY % TEI.prose "IGNORE">
<![ %TEI.prose; [
 .... lots of other declarations ...
<!ENTITY % biblStruct "INCLUDE">
<![ %biblStruct; [
  <!ELEMENT biblStruct .... >
  <!ATTLIST biblStruct .... >
]]> … yet more declarations
 ]]>
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