text: <!DOCTYPE doc [
  <!ATTLIST doc attrExtEnt CDATA #IMPLIED>
  <!ENTITY ent1 "Hello">
  <!ENTITY ent2 SYSTEM "world.txt">
  <!ENTITY entExt SYSTEM "earth.gif" NDATA gif>
  <!NOTATION gif SYSTEM "viewgif.exe">
  ]>
  <doc attrExtEnt="entExt">
     &ent1;, &ent2;!
  </doc>
  
  <!-- Let world.txt contain "world" (excluding the quotes) -->
  