Name
Proper Declaration/PE Nesting
Text from specification
Parameter entity replacement text must be properly nested with markup declarations. That is to say, if either the first character or the last character of a markup declaration is contained in the replacement text for a parameter entity reference, both must be contained in the same replacement text.
Explanation
This constraint means you can’t create a parameter entity that completes one DTD declaration and begins another; the following XML fragment would violate this constraint:
<!ENTITY % finish_it ">"> <!ENTITY % bad "won't work" %finish_it; <!--wrong!-->
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