I am guessing that in SGML a bare "--" can be considered markup.
SGML comments can be embedded inside decarations so
<!ENTITY nbsp " " -- a non breaking space -->
for example.
or even
<!ENTITY -- a keyword -- nbsp -- a name --" " -- a non breaking space -->
The classic one is
<!---------!>
which is legal SGML comment if there is a multiple of four - as if there
are 4n - then this is an empty declaration with n empty comments.
Otherwise it's an error as the declaration would be ill formed.
XML banned nested comments so you have to have
<!ENTITY nbsp " "--><!-- non breaking space -->
but banned -- appearing in comments to ensure that every legal XML
comment
was a legal SGML one, and more generally to ensure every XML document
was an (WWW-)SGML one.
David
XSL-List info and archive: http://www.mulberrytech.com/xsl/xsl-list