built-in shelf
A built-in stream shelf that provides specific information about the input data being parsed in the case of an error. The information provided to #additional-info comes from the markup parser error message. #additional-info can only be accessed from the header and body of sgml-error or markup-error rules.
The information provided is the text of the error message that gives information about this particular error. #recovery-info provides information on how the markup parser will recover from the error and continue processing.
#additional-info is not always attached. Use the is attached test to ensure the stream is attached before attempting to use its value (the information about the input data).