1  <!DOCTYPE doc [
 2  <!ELEMENT doc (a, b, c, b, c, d) >
 3  <!ELEMENT a EMPTY>
 4  <!ELEMENT b EMPTY>
 5  <!ELEMENT c EMPTY>
 6  <!ELEMENT d EMPTY>
 7  <!ENTITY c "<c/>">
 8  <!ENTITY bc "<b/>&c;">
 9  ]>
10  <doc>
11  <a/>
12  &bc;
13  &bc;
14  <d/>
15  </doc>