• SET-DISPATCH-MACRO-CHARACTER

CL-USER(1): (set-dispatch-macro-character #\# #\; (lambda (s c n) c n (read s t nil t) (values)))

CL-USER(2): (+ 3 (+ 2 3) 1)

9
CL-USER(3): (+ 3 #;(+ 2 3) 1)

4

Special thanks to Ed Langley on #common-lisp:matrix.org.