built-in shelf
#appinfo is a built-in, read-only stream that provides the value of the appinfo
parameter of an SGML declaration.
Whether the #appinfo shelf has a value depends on the state of SGML parsing. It has a value only if
the SGML declaration has already been parsed and it had an appinfo declaration other than none. If this is not the case, it is unattached. Therefore, #appinfo is
never attached in
document-start rule,
do sgml-parse block, prior to the execution of the first %c or suppress,
unless a pre-compiled DTD is used,
document-end rule, when errors occurred before this information could be determined.
If an SGML declaration was not given, or if appinfo none was specified in the SGML
declaration, #appinfo is unattached.