WHILE TRUE DO
    keys$ := get_keys$
    IF keys$ = "" DO keys$ := "<none>"
    IF keys$ <> old_keys$ THEN
        CLS
        PRINT "The following keys are down:";''keys$
    ENDIF
    old_keys$ := keys$
ENDWHILE

FUNC get_keys$ CLOSED
    SYNC
    keys$ := ""
    FOR n% := 0 TO 127 DO
        IF KEY_DOWN n% THEN
            IF LEN key$ <> 0 DO key$ := key$ + ", "
            key$ := key$ + STR$ n%
        ENDIF
    NEXT n%
    RETURN key$
ENDFUNC get_keys$