This run of filled squares is already as long as any block that could sit here, so both ends are empty.
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When to look for it
When a run of filled squares is as long as the clue that could cover it, and you want to close it off.
Why it holds
Work out which blocks could reach this run at all. If every one of them is exactly as long as the run already is, the run cannot grow — so the square at each end is empty.
Try it
Mark every square this rule settles, then check.
Click a square once to fill it, twice to cross it off, three times to clear it.
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