$675f ld a, ($720d)
$6762 cp $04            ;is this level 4?
$6764 jr z, $66fb       ;yes it is so exit to main game loop as no further
                        ;progress is possible.

                        ;when you touch a graphic $31 or $32 (these
                        ;two graphics represent the exit doors when
                        ;you collect a stone) or a graphic $29 or $2A
                        ;(these two graphics represent part of the
                        ;mine shaft) then a flag at $831d is set to 1.
                        ;see $8353 onwards.

$6766 ld a, ($831d)
$6769 cp $00            ;have we touched an exit?
$676b jr z, $66fb       ;no we haven't so exit to main game loop
$676d jp $6ff8          ;jump to mine carts/load next level etc.