1<?xml version="1.0"?> 2<!DOCTYPE billion [ 3 <!ELEMENT billion (#PCDATA)> 4 <!ENTITY % p0 SYSTEM "p0.ent"> 5 <!ENTITY % p1 SYSTEM "p1.ent"> 6 <!ENTITY % p2 SYSTEM "p2.ent"> 7 <!ENTITY % p3 SYSTEM "p3.ent"> 8 %p3; 9]> 10<billion/> 11