ANSI Common Lisp   22 Printer   22.3 Formatted Output   22.3.4 FORMAT Printer Operations

22.3.4.2 Tilde S: Standard

This is just like ~A, but arg is printed with escape characters (as by prin1 rather than princ). The output is therefore suitable for input to read. ~S accepts all the arguments and modifiers that ~A does.

~S binds *print-escape* to t.