ANSI Common Lisp 9 Conditions 9.1 Condition System Concepts
9.1.3 Printing Conditions
If the :report argument to define-condition is used,
a print function is defined that is called whenever
the defined condition is printed while the value of *print-escape* is false.
This function is called the condition reporter;
the text which it outputs is called a report message.
When a condition is printed and *print-escape*
is false, the condition reporter for the condition is invoked.
Conditions are printed automatically by functions such as
invoke-debugger, break, and warn.
When *print-escape* is true, the object should print in an
abbreviated fashion according to the style of the implementation
(e.g., by print-unreadable-object). It is not required that a
condition can be recreated by reading its printed representation.
No function is provided for directly accessing
or invoking condition reporters.
188.8.131.52 Recommended Style in Condition Reporting