1 <!DOCTYPE doc

     2 [

     3 <!ELEMENT doc (#PCDATA)>

     4 <!ELEMENT a (#PCDATA|doc)*>

     5 <!ELEMENT b (

     6 #PCDATA

     7 |

     8 doc

     9 |

    10 a

    11 |

    12 b

    13 )*>

    14 <!ELEMENT c (#PCDATA)*>

    15 ]>

    16 <doc/>