A mart's buy list scrolls. The cursor walks rows 0, 1, 2 and then the window
moves under it, so an item's position in the counter's stock is its cursor index
only for the first three entries; the offset that would name the rest is not a
pinned address. Pewter's counter carries seven items and the Antidote is its
fourth, so `BUY ANTIDOTE` there aimed a cursor step above the list's own max and
reported `blocked` on its first frame, having pressed nothing -- and a purchase
has no walk target, so nothing was recorded and the button came back on the next
hold.
One accessor answers "which index, if any" for both the pad and the plan, and the
clerk talking is refused through the seam rather than here.
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