ion declaration  [82] NotationDecl ::=
          '<!NOTATION' S Name S (ExternalID |  PublicID) S? '>' 
          Hence there is actually 3 choices: 'PUBLIC' S PubidLiteral
          'PUBLIC' S PubidLiteral S SystemLiteral and 'SYSTEM' S
           SystemLiteral  See the NOTE on xmlParseExternalID(). N)