* "Implementors are encouraged to make sure that there is always a restart named abort around any user code so that user code can call abort at any time ... in some batch or multi-processing situations there may be situations in which having it kill the running process is more appropriate." http://clhs.lisp.se/Body/r_abort.htm
* There's nothing to do inside the debugger with no restarts; the 'a' key in SLIME doesn't work.
* Since bordeaux-threads does not permit a thread to kill itself, there is no implementation-neutral way of self-terminating a thread outside of invoking ABORT (or establishing some other restart beforehand).
* User code which has already added an ABORT restart is OK, since the user's ABORT will hide the new topmost ABORT.
* Peer pressure: these implementations define ABORT inside threads: ABCL, Allegro, Clozure, ECL, LispWorks, SBCL.