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Contradiction Testing
Assuming one cell is true, following the consequences through, and treating a contradiction as proof the assumption was false.
Why it works
If assuming something true leads to two conflicting marks on the same cell, that assumption can't hold, which makes the opposite mark certain without ever needing a clue to say so directly.
Trigger
A genuine stall, after the grid sweep, the ledger sweep, and a re-read of every dynamic clue have all been exhausted.
Common mistake
Reaching for this before actually exhausting the cheaper techniques — it is a last resort, not a shortcut.
Example
Assume Mina has the llama, follow every consequence through, and if that assumption eventually forces two different pets onto the same person, the llama assumption was wrong.
On Bambleweeny's board
Not implemented in the live solver — every Bambleweeny puzzle is built to be solvable by pure deduction, without this. It exists for puzzles built less carefully than these.