Techniques › 4 of 9
Forcing (Edge & Wall Extension)
Level 2: Medium
A filled cell against an edge or a wall forces its block to grow away from the boundary.
A filled cell on the very first position must be the start of the first block. With a clue of 4, that block has to run through cells 1 to 4, so the next three cells are filled.
The same holds next to a crossed-out cell: the wall acts like the edge of the board.