1 <!DOCTYPE doc

     2 [

     3 <!ELEMENT doc ANY>

     4 <!--NOTE: XML doesn't specify whether this is a choice or a seq-->

     5 <!ELEMENT a (doc?)>

     6 <!ELEMENT b (doc,a)>

     7 <!ELEMENT c (

     8 doc

     9 ,

    10 a

    11 ,

    12 c?

    13 )>

    14 ]>

    15 <doc/>