Represents notations like
<!NOTATION PNG SYSTEM "http://www.w3.org/TR/REC-png">
<!NOTATION PS SYSTEM "PS">
<!NOTATION SGML PUBLIC
"ISO 8879:1986//NOTATION Standard Generalized Markup Language//EN">
<!NOTATION TBL SYSTEM "TBL">
<!NOTATION TEX PUBLIC
"+//ISBN 0-201-13448-9::Knuth//NOTATION The TeXbook//EN">
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