  Let $r$ be a positive integer. An $r$-set is a pair $X= (V(X),R(X))$
consisting of a set $V(X)$ with a subset $R(X)$ of the direct product $V(X)^r$.
The object of this paper is to investigate the Hom complexes of $r$-sets, which
were introduced for graphs in the context of the graph coloring problem.
  In the first part, we introduce simplicial sets which we call singular
complexes, and show that singular complexes and Hom complexes are naturally
homotopy equivalent. The second part is devoted to the generalization of
$\times$-homotopy theory established by Dochtermann. We show the folding
theorem for hypergraphs which was partly proved by Iriye and Kishimoto.
