Techniques / Last Resort
Templates (for computers)
All ways to place one digit in the whole grid, checked against what is known.
A template is one way to place a digit once in every row, column and box: there are 46,656 of them. For each digit, discard the templates that use a cell where the digit is not allowed or that miss a cell where it is already placed. If every remaining template of a digit contains a cell, the digit goes there (Template Set). If no remaining template contains a cell, the digit can be removed from it (Template Delete).
The solver can also drop templates that overlap a cell which another digit needs for all its templates, and repeat. This is a computer-sized technique that people rarely use by hand. In HoDoKu it is a last resort before trial and error, and it is switched off by default.
Notation
Template Set: .
pattern
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