<!DOCTYPE bar [
  <!ELEMENT bar - - (foo)>
  <!ELEMENT foo O O (a+)>
  <!ELEMENT a   O O (b,c)>
  <!ELEMENT b   - O (#PCDATA)>
  <!ELEMENT c   - O (#PCDATA)>
]>

<bar>
<b>x<c>y
<b>x<c>y
<b>x<c>y
<b>x<c>y
</bar>
