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A verity is a statement which is true for all alternatives in a limited set.
Suppose we have a cell A and as cell B. Consider the following implication chains:
A=1 => ... => B=2 A<>1 => ... => B=2
Cell A can either contain digit 1 or it does not. There is no middle ground. When both situations lead to the placement of digit 2 in cell B, this cell must contain digit 2. This is a verity.
The alternative term veracity is sometimes used for similar purposes, but there is no real distinction between these two terms.
The following situation shows how a verity can also be proven when there are more than 2 alternatives:
A=(1,2,3) A=1 => ... => B=2 A=2 => ... => B=2 A=3 => ... => B=2
In this sample, cell A has candidates for digits 1,2 and 3. Each of these force digit 2 in cell B.