1<?xml version="1.0"?> 2<!DOCTYPE billion [ 3 <!ELEMENT billion (#PCDATA)> 4 <!ENTITY g0 SYSTEM "g0.ent"> 5 <!ENTITY g1 SYSTEM "g1.ent"> 6 <!ENTITY g2 SYSTEM "g2.ent"> 7 <!ENTITY g3 SYSTEM "g3.ent"> 8]> 9<billion>&g3;</billion> 10