1 <!DOCTYPE doc [ 2 <!ELEMENT doc ((a | b) , (c | d))+> 3 <!ELEMENT a (#PCDATA)> 4 <!ELEMENT b (#PCDATA)> 5 <!ELEMENT c (#PCDATA)> 6 <!ELEMENT d (#PCDATA)> 7 ]> 8 <doc><b>This</b><c> is a valid document</c></doc> 9
