Almost Locked Sets (ALS)
An Almost Locked Set is a group of N cells in a single house with candidates for N+1 digits. In other words, it is one cell short from being a locked set.
The simplest ALS is a single cell with 2 candidates. In a standard Sudoku an ALS can have a maximum size of 8 cells with 9 digits.
It is not easy to spot a large ALS, but there are often a lot of these sets present in the grid. Bivalue cells are always easy to spot with pencilmarks.
There are several solving techniques that make use of Almost Locked Sets. Any move that would eliminate all candidates for a particular digit from the ALS automatically forces the remaining digits to be locked in the group of cells. This the equivalent of a strong link, which can be used in chains and loops.
The ALS-XZ technique uses only 2 Almost Locked Sets to perform eliminations. It is the most basic ALS technique.ALS-XZ rule
2 Almost Locked Sets with restricted common digit X perform eliminations for common digit Z.
3 Almost Locked Sets with 2 restricted common digits Y and Z perform eliminations for common digit X.
A stem cell of N candidates pointing to N petals, each an Almost Locked Set.
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- Chains and Loops
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- Almost Locked Sets