<!DOCTYPE main [

<!ELEMENT a (#PCDATA) >
<!ELEMENT b (#PCDATA) >
<!ELEMENT c (#PCDATA) >
<!-- <!ELEMENT main ( (a, b) | (a, c) ) > -->
<!ELEMENT main  (a, (b|c)  )>
]>
<main>
   <!-- Content model is non-deterministic, so this will fail even
   though it seems to make sense -->
   <a> string </a>
   <c> string </c>
</main>

