ANSI Common Lisp 22 Printer 22.1 The Lisp Printer 22.1.3 Default Print-Object Methods
220.127.116.11 Printing Other Objects
Other objects are printed in an implementation-dependent manner.
It is not required that an implementation print those objects
For example, hash tables,
might not print readably.
A common notation to use in this circumstance is #<...>.
Since #< is not readable by the Lisp reader,
the precise format of the text which follows is not important,
but a common format to use is that provided by the print-unreadable-object macro.
For information on how the Lisp reader treats this notation,
see Section 18.104.22.168 Sharpsign Less-Than-Sign.
For information on how to notate objects that cannot be printed readably,
see Section 22.214.171.124 Sharpsign Dot.