In Common Lisp you can have namespaces, where you can have your own SETQ: CL-USER 53 > (defpackage mylisp (:use "COMMON-LISP") ; everythi

In Common Lisp you can have namespaces, where you can have your own SETQ: CL-USER 53 > (defpackage mylisp (:use "COMMON-LISP") ; everything from Common Lisp (:shadow CL:SETQ)) ; but not SETQ #<The MYLISP package, 1/16 internal, 0/16 external> CL-USER 54 > (in-package "MYLISP") #<The MYLISP package, 1/16 internal, 0/16 external> MYLISP 55 > (defun setq (a b c) (print (list a b c))) SETQ MYLISP 56 > (setq (+ 1 2) 4 5) (3 4 5) (3 4 5) MYLISP 57 > (cl:setq a 12) 12 Most implementations also have a way to protect core symbols against redefinition and a way to do it anyway (possibly shooting yourself into the foot).