Term for edges between graph homomorphisms?

If we define a graph to be a tuple (E, V, s: E -> V, t: E -> V), then the category Gph of graphs and graph homomorphisms is cartesian closed (in fact, a topos). For any pair of graphs G, G', there is a "hom graph" whose vertices are graph homomorphisms from G to G' and whose edges are things I've been calling "graph shifts". A graph shift S between two graph homomorphisms F, F':G -> G' assigns to each vertex g in G an edge S(g) in G' from F(g) to F'(g). Is there a more common term for a "graph shift"? -- Mike Stay - metaweta@gmail.com http://www.cs.auckland.ac.nz/~mike http://reperiendi.wordpress.com [For admin and other information see: http://www.mta.ca/~cat-dist/ ]