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Mercury (Bouncing)
Level 3: Hard
A block that would run into a filled cell right after it cannot start next to the wall, so it bounces away.
The 3-block cannot start right after the cross at position 1: it would occupy cells 2 to 4 and touch the filled cell at 5, making a block of 4. Cell 2 is therefore empty.
Think of the block as a drop of mercury that slides away from a wall when the space ahead is not the right size.