break when encountering a hard or soft error caused by ACL2
Major Section:  TRACE

General forms:
(break-on-error t)   ; installs a trace causing a continuable error (break)
                     ;   whenever an error is invoked by ACL2.
(break-on-error)     ; same as above
(break-on-error nil) ; uninstall the above trace
(Break-on-error) generates roughly the following in raw Lisp:
(trace (error1 :entry nil :exit (break)))
This trace should cause entry to the Lisp debugger (at least in most Lisps) whenever ACL2 calls its error routines.

NOTE: Break-on-error causes a hard error if the underlying Lisp does not comprehend the :entry and :exit keywords for trace$. As of this writing, break-on-error works in OpenMCL, GCL, and Allegro CL.

Also see trace$.