
theorem :: this would otherwise need Proof outside of this article
  for G1, G2, G3 being _Graph
  for F1 being PGraphMapping of G1, G2, F2 being PGraphMapping of G2, G3
  holds (F2*F1)_V = F2_V * F1_V & (F2*F1)_E = F2_E * F1_E;
