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><d> is a valid document</d></doc> 9
