(define a 3)
(define define +) #makes define useless
(define a 1) #outputs 4, instead of assigning 1 to a.

(define + -)
(+ 1 1) #Output 0.

CL-USER 1 > (defun defun () 3)

Error: Redefining function DEFUN visible from package COMMON-LISP.
  1 (continue) Redefine it anyway.
  2 (abort) Return to level 0.
  3 Restart top-level loop.

* (defun defun () 3)

debugger invoked on a SYMBOL-PACKAGE-LOCKED-ERROR:
  Lock on package COMMON-LISP violated when proclaiming DEFUN as a function
  while in package COMMON-LISP-USER.
See also:
  The SBCL Manual, Node "Package Locks"
  The ANSI Standard, Section 11.1.2.1.2

Type HELP for debugger help, or (SB-EXT:EXIT) to exit from SBCL.

restarts (invokable by number or by possibly-abbreviated name):
  0: [CONTINUE      ] Ignore the package lock.
  1: [IGNORE-ALL    ] Ignore all package locks in the context of this operation.
  2: [UNLOCK-PACKAGE] Unlock the package.
  3: [ABORT         ] Exit debugger, returning to top level.