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