= Constraint propagation
<Constraint propagation> repeatedly removes variable values incompatible with local consequences of a <constraint satisfaction problem>. The goal is to reduce the remaining search while preserving all genuine solutions. For $X<Y$ with domains $\{1,2\}$, enforcing support removes two from $X$ and one from $Y$, leaving the forced pair $(1,2)$. <Arc consistency> checks support across binary constraints; dedicated global-constraint propagators can reason about larger <sets> together. Reaching a propagation fixed point does not generally prove that the whole problem has a solution, because locally supported choices can still conflict globally.
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