+ DEFVAR_LISP ("debug-on-event",
+ Vdebug_on_event,
+ doc: /* Enter debugger on this event. When Emacs
+receives the special event specifed by this variable, it will try to
+break into the debugger as soon as possible instead of processing the
+event normally through `special-event-map'.
+
+Currently, the only supported values for this
+variable are `sigusr1' and `sigusr2'. */);
+ Vdebug_on_event = intern_c_string ("sigusr2");
+