<?xml version="1.0"?>
<!DOCTYPE billion [
  <!ELEMENT billion (#PCDATA)>
  <!ENTITY g0 SYSTEM "g0.ent">
  <!ENTITY g1 SYSTEM "g1.ent">
  <!ENTITY g2 SYSTEM "g2.ent">
  <!ENTITY g3 SYSTEM "g3.ent">
]>
<billion>&g3;</billion>