DEBUG expression

   Synopsis:
      Writes a 'debug' message to the 'main' window

   Notes:
      The message is also written to Axmud's logfiles (if logging is enabled).
      See also the help for ERROR, PRINT, WARNING and WRITE.

   Examples:
      ! Get a number between 1 and 6
      LET number = Int (Rnd (6) + 1)

      ! The subroutine will never be called
      IF number = 0 THEN CALL Terminate()
      END

      SUB STRING Terminate ()
         DEBUG "This subroutine should not have been called!"
         STOP
      END SUB
