For graphs "G" and "H", the question of whether "G" has a homomorphism to "H" corresponds to a CSP instance with only one kind of constraint, as follows. The "variables" are the vertices of "G" and the "domain" for each variable is the vertex set of "H". An "evaluation" is a function that assigns to each variable an element of the domain, so a function "f" from "V"("G") to "V"("H"). Each edge or arc ("u","v") of "G" then corresponds to the "constraint" (("u","v"), E("H")). This is a constraint expressing that the evaluation should map the arc ("u","v") to a pair ("f"("u"),"f"("v")) that is in the relation "E"("H"), that is, to an arc of "H". A solution to the CSP is an evaluation that respects all constraints, so it is exactly a homomorphism from "G" to "H".