CL-USER> (defun @-processor (stream char)
           'oh-noes)
;Compiler warnings :
;   In @-PROCESSOR: Unused lexical variable CHAR
;   In @-PROCESSOR: Unused lexical variable STREAM
@-PROCESSOR
CL-USER> (set-macro-character #\@ #'@-processor)
T
CL-USER> '(@ a b c)
(OH-NOES A B C)
CL-USER> (setf *readtable* (copy-readtable nil))
#<READTABLE #x302000D8048D>
CL-USER> '(@ a b c)
(@ A B C)