Constant counts transfer between labellings agreeing on the window.
Constant adjacency to a vertex on a window gives constant counts into its singleton set.
The two-pointer pass separates a window: the first cnt positions
hold splitter-adjacent vertices and the remainder non-adjacent ones,
where cnt is the window's adjacency count.
A processed cell of the trivial-splitter pass: the labelling outside the cell is untouched, the cell keeps its contents as a multiset, and it is rearranged into the splitter-adjacent block followed by the non-adjacent block.
Adjacency to a vertex constant over a member list gives constant counts into its singleton set, through row symmetry.
The split bookkeeping's partition effect: one boundary at the
final c2 when the split is nontrivial, nothing otherwise.
One processed cell of the trivial pass: sizes and the outside kept, and the window left as constant block(s) with the junction boundary written exactly in the two-block case.
The trivial pass over a window list: sizes kept, everything outside the windows kept, and every window left as constant block(s) with a surviving junction boundary in the two-block case.
After the trivial pass, every cell of the result within range is adjacency-constant to the captured splitter row.
The scan closes the junction after each nonempty group that ends inside the cell.
One processed cell of the nontrivial pass: sizes and the outside kept, and positions of the window connected by an open run of the result carry equal counts into the captured splitter set.
The nontrivial pass over a window list: sizes and the outside kept, and within every window, positions connected by an open run of the result carry equal counts into the captured splitter set.
After the nontrivial pass, every cell of the result within range has constant counts into the captured splitter set.
After one refineStep, every cell of the result within range has
constant counts into the retired splitter's captured vertex set.
With no active cells the active union vanishes.