We consider a graph polynomial ξ(G;x, y, z) introduced by Ilia Averbouch, Benny Godlin, and Johann A. Makowsky (2008). This graph polynomial simultaneously generalizes the Tutte polynomial as well as a bivariate chromatic polynomial defined by Klaus Dohmen, André Pönitz, and Peter Tittmann (2003). We derive an identity which relates the graph polynomial ξ of a thickened graph (i.e. a graph with...