
theorem
  for G1 being _Graph, G2 being G1-isomorphic _Graph
  for G3 being G2-isomorphic _Graph, F being Isomorphism of G1, G2
  st G1 == G3 holds F" is Isomorphism of G2, G3
proof
  let G1 be _Graph, G2 be G1-isomorphic _Graph;
  let G3 be G2-isomorphic _Graph, F being Isomorphism of G1, G2;
  assume A1: G1 == G3;
  G1 is reverseEdgeDirections of G1, {} by GLIB_009:43;
  then G3 is reverseEdgeDirections of G1, {} by A1, GLIB_007:2;
  hence thesis by Th97;
end;
