Subset deduction

Level: Intermediate

If one number's candidates fall inside another's, use the difference.

This technique compares the 'set of closed neighbors' of two adjacent number cells. It holds when one cell's set of mine candidates is entirely contained within the other cell's candidate set.

For example, if A's candidates are contained in B's, and B's required mine count minus A's mine count equals the 'number of cells in the difference,' then that difference set is all mines. Conversely, if that value is 0, they're all safe.

The 1-1 and 1-2 patterns are, in fact, special cases of this subset logic. Thinking in terms of set containment lets you handle even complex borders in a general way.

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