“…Let P G be a ρ(G − )-set, let P H be a ρ(H)-set, and let I be the set of all singletons of G. For a vertex v ∈ V (H), we set P = (P G ×{v})∪(I ×P H ). According to (8), and the paragraph after it, we can deduce that for any two vertices (g, h), (g ′ , h ′ ) ∈ P , it follows that d G•H ((g, h), (g ′ , h ′ )) ≥ 3.…”