#message

A built-in stream shelf that provides the text of the fixed part of an SGML or XML error message. This stream shelf can be accessed only from the header and body of sgml-error rules.

#message also can be used as a pattern variable for compatibility with past releases. However, this use is deprecated. Unlike programmer-defined pattern variables, #messagemust always be heralded with the pattern keyword, even where heralding is usually optional.

The is specified test is always true for #message because the fixed part is present in all SGML error messages.