collapseos/blk/636

: (key)
    _next C@ IF _next C@ 0 _next C! EXIT THEN
    BEGIN _updsel UNTIL
    _prevstat C@
    0x20 ( BUTC ) OVER AND NOT IF DROP _sel C@ EXIT THEN
    0x40 ( BUTA ) AND NOT IF 0x8 ( BS ) EXIT THEN
    ( If not BUTC or BUTA, it has to be START )
    0xd _next C! _sel C@
;