
theorem Th110:
  for G1, G2, G3 being _Graph, F1 being PGraphMapping of G1, G2,
    F2 being PGraphMapping of G2, G3 st F1 is Disomorphism & F2 is Disomorphism
  holds F2 * F1 is Disomorphism
proof
  let G1, G2, G3 be _Graph;
  let F1 be PGraphMapping of G1, G2, F2 be PGraphMapping of G2, G3;
  assume F1 is Disomorphism & F2 is Disomorphism;
  then F2 * F1 is directed isomorphism by Th109;
  hence thesis;
end;
