Primal graph
= Primal graph
The <graph> with one <vertex> per <variable> and an <edge> between distinct <variables> occurring in a common constraint scope. Each <hypergraph edge> of the <constraint hypergraph> becomes a <clique>. A <constraint satisfaction problem> consisting only of <all-different constraints> on a common <constraint domain> is exactly a <proper graph colouring> problem on this <primal graph>.