reserve G, G2 for _Graph, V, E for set,
  v for object;

theorem Th5:
  for G2 for G1 being reverseEdgeDirections of G2
  holds G2 is reverseEdgeDirections of G1
proof
  let G2;
  let G1 be reverseEdgeDirections of G2;
  the_Edges_of G1 = the_Edges_of G2 by Th4;
  hence thesis by Th3;
end;
