### 3.7.80. Difference between pairs of variables

A constraint that allows expressing the fact that a set of pairs of variables are different. Two pairs of variables $\left({X}_{1},{Y}_{1}\right)$ and $\left({X}_{2},{Y}_{2}\right)$ are different if and only if ${X}_{1}\ne {X}_{2}$ or ${Y}_{1}\ne {Y}_{2}$.