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4.5.3 Extra Condition in Regina
Regina has some other extra conditions. These conditions are:

    A condition DEBUG, that is very similar to the condition
  HALT. The condition is raised as a result of an event of external
  origin, generally a special combination of keystrokes is pressed.

    The default-action of this condition is to set the trace mode
  to Normal and interactive. Consequently, the user will generally
  get into interactive tracing at the next clause boundary. This
  way, the user may be able to stop the program during execution,
  and perform debugging.

    The delay-action of this condition is to ignore it.

    On Unix machines, this is the signal QUIT (SIGQUIT), which is
  normally bound the <ctrl>-\ key.  Just like condition HALT, this
  might also be simulated from the command line, or from other
  programs. Consult the Unix documentation for more information. On
  VAX/VMS machines, this event is normally bound to the <ctrl>-<y>

    This extended condition will only be available if the
  extension COND_DEBUG has been chosen.

Whether or not the conditions listed here are available, may also
depend on whether particular preprocessor flag was set during
compilation. For more information, see the chapter on extensions.