REDEF!

system hacker's redefinition command
Major Section:  MISCELLANEOUS

Example and General Form:
ACL2 !>:redef!
ACL2 p!>
This command sets ld-redefinition-action to '(:warn! . :overwrite) sets the default defun-mode to :program, and invokes (set-state-ok t). It also introduces (defttag :redef!), so that redefinition of system functions will be permitted; see defttag.

This command is intended for those who are modifying ACL2 source code definitions. Thus, note that even system functions can be redefined with a mere warning. Be careful!