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<!DOCTYPE doc
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[
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<!ELEMENT doc (a|b)*>
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<!ELEMENT a ANY>
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<!ELEMENT b ANY>
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<!NOTATION a SYSTEM "a">
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<!ATTLIST a att (a|b) #IMPLIED>
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<!ATTLIST b att NOTATION (a|b) #IMPLIED>
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<!NOTATION b SYSTEM "b">
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]>
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<doc/>