This entity rules out things like:
<!ENTITY % p1 "(A|B">
<!ENTITY % p2 "|C|D)">
<!ELEMENT X %p1;%p2;>
This makes life easier for people who write XML editors and other authoring software tools; managing parameter entities is tough enough for these people without having them spill all over the place.
Copyright © 1998, Tim Bray. All rights reserved.