Cause an immediate Lisp break
ACL2 users are generally advised to avoid breaking into raw Lisp.
Advanced users may, on occasion, see the need to do so. Evaluating
(break$) will have that effect. (Exception: break$ is disabled
after evaluation of (set-debugger-enable :never); see set-debugger-enable.) Break$ returns nil.
(defun break$ nil (declare (xargs :guard t))