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<?xml version="1.0"?>
<!DOCTYPE billion [
  <!ELEMENT billion (#PCDATA)>
  <!ENTITY % p0 SYSTEM "p0.ent">
  <!ENTITY % p1 SYSTEM "p1.ent">
  <!ENTITY % p2 SYSTEM "p2.ent">
  <!ENTITY % p3 SYSTEM "p3.ent">
  %p3;
]>
<billion/>