control whether Lisp errors and breaks invoke the Lisp debugger

Forms (see below for explanations and GCL exceptions):

(set-debugger-enable t)         ; enable breaks into the raw Lisp debugger
(set-debugger-enable :break)    ; same as above
:set-debugger-enable t          ; same as above
(set-debugger-enable :break-bt) ; as above, but print a backtrace first
(set-debugger-enable :bt-break) ; as above, but print a backtrace first
(set-debugger-enable :bt)       ; print a backtrace but do not enter debugger
(set-debugger-enable :never)    ; disable all breaks into the debugger
(set-debugger-enable nil)       ; disable debugger except when calling break$

Introduction. Suppose we define foo in :program mode to take the car of its argument. This can cause a raw Lisp error. ACL2 will then return control to its top-level loop unless you enable the Lisp debugger, as shown below (except: the error message can be a little different in GCL).

  ACL2 !>(defun foo (x) (declare (xargs :mode :program)) (car x))

  Form:  ( DEFUN FOO ...)
  Rules: NIL
  Warnings:  None
  Time:  0.00 seconds (prove: 0.00, print: 0.00, other: 0.00)
  ACL2 !>(foo 3)
  ************ ABORTING from raw Lisp ***********
  Error:  Attempt to take the car of 3 which is not listp.

  If you didn't cause an explicit interrupt (Control-C),
  then the root cause may be call of a :program mode
  function that has the wrong guard specified, or even no
  guard specified (i.e., an implicit guard of t).
  See :DOC guards.

  To enable breaks into the debugger (also see :DOC acl2-customization):
  ACL2 !>(foo 3)
  Error: Attempt to take the car of 3 which is not listp.
    [condition type: TYPE-ERROR]

  Restart actions (select using :continue):
   0: Abort entirely from this (lisp) process.
  [Current process: Initial Lisp Listener]
  [1] ACL2(1): [RAW LISP] 

Details. ACL2 usage is intended to take place inside the ACL2 read-eval-print loop (see lp). Indeed, in most Lisp implementations ACL2 comes up inside that loop, as evidenced by the prompt:

ACL2 !>
However, one can occasionally hit a raw Lisp error. Here is the above example again, this time for a GCL implementation, which unfortunately gives a slightly less aesthetic report.
  ACL2 !>(foo 3)

  Error: 3 is not of type LIST.
  Fast links are on: do (si::use-fast-links nil) for debugging
  Error signalled by CAR.
  Backtrace: funcall > system:top-level > lisp:lambda-closure > lp > acl2_*1*_acl2::foo > foo > car > system:universal-error-handler > system::break-level-for-acl2 > let* > UNLESS
  ACL2 !>

Here, the user has defined foo in :program mode, with an implicit guard of t. The ACL2 evaluator therefore called the Lisp evaluator, which expected nil or a consp argument to car.

By default, ACL2 will return to its top-level loop (at the same level of LD) when there is a raw Lisp error, as though a call of ER with flag HARD has been evaluated. If instead you want to enter the raw Lisp debugger in such cases, evaluate the following form.

(set-debugger-enable t)
You can subsequently return to the default behavior with:
(set-debugger-enable nil)
Either way, you can enter the Lisp debugger from within the ACL2 loop by evaluating (break$). If you want break$ disabled, then evaluate the following, which disables entry to the Lisp debugger not only for Lisp errors but also when executing (break$).
(set-debugger-enable :never)

The discussion above also applies to interrupts (from Control-C) in some, but not all, host Common Lisps.

It remains to discuss options :break, :bt, :break-bt, and :bt-break. Option :break is synonymous with option t, while option :bt prints a backtrace (except in GCL, where a backtrace is already printed for :break). Options :break-bt and :bt-break are equivalent, and each has the combined effect of :bt and :break: a backtrace is printed and then the debugger is entered.

Note that set-debugger-enable applies not only to raw Lisp errors, but also to ACL2 errors: those affected by break-on-error. However, for ACL2 errors, entering the debugger is controlled only by break-on-error, not by set-debugger-enable; so for ACL2 errors, set-debugger-enable values of :bt, :break-bt, and :bt-break have the same effect (namely, of causing a backtrace to be printed, other than for GCL).

Remark for Common Lisp hackers (except for GCL). You can customize the form of the backtrace printed by entering raw Lisp (with :q) and then redefining function print-call-history, whose definition immediately precedes that of break-on-error in ACL2 source file ld.lisp. Of course, all bets are off when defining any function in raw Lisp, but as a practical matter you are probably fine as long as your books are ultimately certified with an unmodified copy of ACL2. If you come up with improvements to print-call-history, please pass them along to the ACL2 implementors.