 The hypergraph is recovered by defining the vertices as the variable-vertices and the hyperedges as the sets of variable-vertices connected to each constraint-vertex.The primal constraint graph or simply primal graph (also the Gaifman graph) of a constraint satisfaction problem is the graph whose nodes are the variables of the problem and an edge joins a pair of variables if the two variables occur together in a constraint.The primal constraint graph is in fact the primal graph of the constraint hypergraph.