After taking an immediate win or blocking one, Red plays the cell
whose level is the lowest level held by exactly one playable
cell. A level held by two or more playable cells is skipped, never an
error. Every empty cell holds a level; 0 is strongest.
Upstream also accepts a-d below
9; they import and verify, but the palette and the
engine use the ten digits, as the published solution does.
Diagrams and links in the old seven-marker language still load: they
are translated, which plays identically for any diagram that
verified.