No block can stretch this far, wherever it starts, so this square is empty.
2before
2after
When to look for it
When a filled square near one end pins a block down and you want to know how far it can reach.
Why it holds
Push every block as far left as it will go, then as far right. That gives each block the widest stretch it could ever cover. A square outside all of those stretches can never be filled.
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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